Frozen Dessert Science: Toasted Marshmallow Semifreddo

Summer heat is here!  Time for some sweltering days. There is nothing - I mean NOTHING better to end a summer meal than a cold bite of a frozen dessert. Frozen desserts can be tricky. Many chefs will shy away from them for fear of not getting them right and then being left with a lack-luster dessert and wasted hours. However, a flavorful house-made frozen dessert can jump straight to the front of the line of crave-able, praiseworthy dishes.  Keep these things in mind as you craft frozen desserts for your summer menus! Read to the end to get a FREE RECIPE sure to blow your guests away!
The art of dessert making is truly just as much science as it is art.  There are many technical points to keep in mind as you make any dessert item.  For frozen desserts, the key points to remember are fat and water crystallization, temperature, and texture. 
The structure of the final product is always a concern for pastry chefs and bakers. With frozen desserts, the primary focus is the balance of fat and water in the base and how to control crystallization.  The presence of fat/water crystals will determine how smooth or icy the final product is. As in all pastry processes, movement=crystals.  So, putting a mix for sorbet in an ice cream machine will give you a smooth frozen dessert. The same sorbet base poured into a container and frozen without moving it will create a totally different product.  All of the tiny ice crystals that form in the sorbet base while it is churned create the smooth final texture.  When untouched, the base will freeze with fewer crystals forming, creating an icy block that you could scrape into a granita. Both can be desirable outcomes in their own way, but it helps to understand how crystals form to judge the final product.
Fat also forms crystals, which is necessary to understand, because dairy fat is a major component of ice cream products. Here are some things to remember about fats:
1. The cocoa butter in chocolate is HARD at room temperature, so imagine what it will be in a frozen dessert.  Be careful using chocolate in a frozen dessert base. Often it will freeze faster than the rest of the base creating a grainy, sandy final product.  I will typically opt for cocoa powder to get a rich chocolate flavor.
2. Caramel containing very little butter or sugar will not have enough body to hold up in a frozen dessert, because sugar is an anit-freeze (more on that below).  Likewise, a caramel with too much dairy fat (usually butter) will be too hard in a frozen dessert and won't have that classic gooey quality that makes caramel such a great addition to frozen desserts. I typically choose a caramel with an amount of butter that equals 6% the weight of the sugar (1 tbsp per cup of sugar) and cream at about 60% the weight of the sugar (10 tbsp per cup of sugar).  
In cooking, we always want to remember the actions that affect the FLAVOR of our food.  Proper browning of meats and vegetables yields a rich flavor.  Temperature is one of these components of a dish that affects flavor.  In frozen desserts, we see this in interesting ways.  Something that is very flavorful at room temperature might have little to no flavor at temperatures below freezing. Berries are a great example.  Whole berries for our purposes here contain: flavor solids, water and acid.  The water crystallizes in huge chunky crystals because the berries are kept whole.  So when you eat a frozen berry, your mouth first needs to melt the ice (not always a pleasant feeling for the diner) before you can taste the flavor compounds.  The acid is present immediately, so you might get the acid of an icy berry before you taste the flavor.  Ideally, you as the diner experience all of these at once: flavor, acid and texture. 
Other flavor compounds often need to be exaggerated in frozen desserts.  In order to transmit flavor, caramel typically needs to be darker for frozen desserts than when paired with room temperature or warm desserts.
So, when you taste a frozen dessert in the making, blandness doesn't necessarily mean that the usual fixes are the next step.  You might need salt, or acid to cut through the fat to get the flavors.  But it might be that the flavors need to be pulled forward by exaggerating them. Toast nuts a touch more than you might for a salad - still not burning them!!  The recipe below for toasted marshmallow semifreddo needs DARK toasted marshmallows.
Texture in frozen desserts is KEY to enjoyment.  This goes hand in hand with temperature, because the first way to ensure the proper texture is serving the dessert at the proper temperature.  All frozen desserts are best enjoyed at a temperature range of 5-10 degrees Fahrenheit. This allows the product to be frozen but melt quickly enough for you to taste the flavor compounds and the texture of the dessert at the same time.
In addition to the temperature of the dessert, you also want to consider the water and fat in the dessert as mentioned above AS WELL as the sugar content and alcohol.  Sugar and alcohol are antifreeze agents. They will prevent items from solidifying at temperatures below the freezing point of water. So a base with high sugar content will be softer than the same recipe with less sugar. So, taking a vanilla ice cream base and adding butterscotch and scotch whiskey to make butterscotch ice cream will yield a softer product than just using the original vanilla ice cream base.
Lastly on texture, one additional note about water. The water content in a frozen dessert impacts the texture in several ways.  Firstly, as mentioned above, the "free water" or water available to turn into ice when frozen, creates ice crystals and how large or small those are affect the smoothness of the final result. Oftentimes, recipes will include stabilizers that "bind" this water and make it unavailable to turn into ice. There are some natural stabilizers like pectin found in fruit or cinnamon that can accomplish this.
This free water will also MIGRATE in a frozen dessert.  Remember molecules in a solid state still move, just extremely slowly.  So, if you put a crunchy inclusion inside your ice cream like hard caramel shards, nuts, cookies or pretzels, those will eventually soften because they will soak up water as it slowly migrates towards those chunks.  To prevent this, coating these items in a flavorless fat like cocoa butter creates a barrier between the crunchy inclusion and the free water in the ice cream base.
Toasted Marshmallow Semifreddo
Those lucky few equipped with an ice cream machine don't know the let down of not being able to include scratch-made frozen desserts on the menu.  My answer to this - semifreddo. 
Semifreddo is a still frozen dessert, meaning it has air whipped into the base like mousse and then is frozen untouched.  The motion creates small fat crystals and air pockets to create a creamy texture. This preparation typically requires a mixer, but sometimes you can whip everything by hand, so you might not need anything beyond something to freeze the dessert in.  You can pipe the whipped base into glasses (make sure to use tempered glass!) or into molds.  I use silicon or metal.
Special notes on this recipe.  I developed this recipe out of a love for the summer favorite: the s'more.  Make sure to toast the marshmallows until DARK.  I usually use the salamander to toast the marshmallows.  A blow torch is too aggressive.  The outside of most commerically available marshmallows is treated to prevent sticking and that exterior burns too quickly under the flame of a blow torch.  Once the outside is browned, you can spread the marshmallow mixture, exposing the insides and this gooey center can then be toasted with the blow torch.  I look for an almost black level of toast with golden brown on the edges each time I toast. I will toast three times. Toast the outside in the salamander, remove and spread to expose the white interior. Toast that, and then repeat once. The marshmallows are then ready to include in the semifreddo. Remember if the marshmallows are not toasted enough you won't taste that smoky, burnt caramel flavor when they are frozen. Enjoy!!
Toasted Marshmallow Semifreddo
Yield: Twelve 2 oz portions
Ingredient                                                             Amount
Egg yolks                                                               3 ea
Sugar                                                                     2 Tbsp
Marshmallows, very well toasted                           2.5 oz
Egg white                                                               1 ea
Cream, whipped to soft peaks                               1 cup
1. Whip egg yolks and sugar over a double boiler until thick and pale yellow.
2. Add the freshly toasted, warm marshmallows and stir until incorporated. 
3. Whip egg white in a bowl and fold into marshmallows.
4. Fold in the whipped cream.
5. Pipe into molds and freeze for at least 6 hours or overnight.