Prepare the sugar syrup
A small amount of corn syrup is often used to help ensure that the sugar does not crystallize. As you’ll see in the recipe, I like to skip the corn syrup. As long as I heat the granulated sugar and water properly and use the resulting syrup right away, I don’t have any problems. Stir together water, sugar, and corn syrup if you are using it (two tablespoons added to my recipe should do it), in a saucepan and bring to a boil over medium-high heat. Once the sugar is dissolved, no more stirring—you don’t want to create any undesirable sugar crystals. At the very most, give the pan a little swish to distribute the heat evenly. Once the syrup has been boiling for about 5 minutes, it should be at a temperature of 240°F.
Bloom the gelatin
While the syrup is heating up, combine water and powdered gelatin in the bowl of your stand mixer, or whatever deep bowl you’ll eventually use to beat the ingredients together.
The protein in the gelatin is key to getting the mixture to whip up once you beat the sugar syrup in. Simply substituting a vegetable gelatin, which is mostly starch, will not work.
Add syrup to gelatin
Stir the mixture until the gelatin is fully dissolved. Then slowly crank up the speed of your mixer and beat the mixture until it's thick, shiny, marshmallow-y, and almost tripled in volume.
This is when having both a stand mixer and an electric hand mixer is really useful. While the marshmallow fluff is fluffing up, beat the egg whites in a separate bowl if you are using them.
When the mixture is almost done, drizzle in extracts. Sift in powdered flavoring like cocoa or matcha with the mixer at low speed.
This is what the beaten syrup/gelatin mixture should look like when it is ready. At this point you can either spread it into a pan and let it set or add egg whites to it.
I like the eggless marshmallow version for toasting and for hot cocoa – it holds its shape better. When eating marshmallows straight-up, I like the airiness of the ones with egg whites.
Beat the egg whites (optional)
You can skip the egg whites altogether and you’ll end up with a slightly firmer, but delicious marshmallow.
If you are using egg whites, beat them until stiff peaks form. You can also beat reconstituted powdered egg whites. Just don’t use pasteurized egg whites from the carton; they’ll never form the peaks that you need.
Add the beaten egg whites to the marshmallow fluff
Blend just until combined. Switching to a paddle attachment on the stand mixer would make the job easier.
Spread into a prepared pan and allow to set
The pan that you spread the marshmallow into should be prepared in advance by spreading a thin coating of butter or oil on the inner surfaces and then dusting them with a mixture of equal parts confectioners sugar and potato or corn starch.
Turn out and cut
Let it set overnight. Run a small offset spatula around the edges and turn the marshmallow out onto a surface that has been dusted with the sugar-starch powder.
Use cookie cutters, scissors, or a sharp chef’s knife to cut the marshmallow into pieces. If you use a knife, press the knife down into the marshmallow – it will bounce back. Don’t saw back and forth.
Piping out marshmallows
You can also use a pastry tube for making Peeps-inspired marshmallow shapes (just dip them in superfine sugar instead of the sugar-starch powder). For easy mini marshmallows, use a pastry tube fitted with a plain tip to make long strips of marshmallow. When it is set, use a scissor to cut the strips down into small pieces.