Thick and flavorful homemade marshmallow creme (aka marshmallow fluff) is easy to make. Use it for s'mores, as a frosting base for cakes and cupcakes, or as an ice cream mix-in. It's better than store bought!
In the bowl of a stand mixer fitted with a whisk attachment, combine the egg whites and cream of tartar. Let this sit while you go to make the sugar syrup.
Combine the water, sugar, and corn syrup in a 3-quart saucepan, stirring to wet the sugar. Clip a candy thermometer to the side of the pan, making sure it's in the mixture but not touching the bottom. Bring the mixture to a boil over medium-high heat without stirring.
As the sugar syrup begins to boil (the temperature will be at about 225 °F), beat the egg whites on high speed just to soft peaks, about 3 to 4 minutes. Keep an eye on the sugar syrup - it should reach 240 °F (soft-ball stage) just about the same time the egg whites are ready.
Turn the mixer to low and slowly pour the hot sugar syrup in a thin stream down the side of the bowl. Don't worry if the egg whites lose some volume at first - they will recover as the sugar syrup gets beaten in.
After all the syrup is added, turn the mixture to high and whip for 7 to 8 minutes. The marshmallow creme will be very thick and glossy at this stage. Add the vanilla and whip for 1 more minute. That's all there is to do!
Storage instructions: Transfer the marshmallow creme into an airtight container and store at room temperature. It will keep up to six weeks. This recipe can be easily doubled.
Cleaning tip: When it's time to clean your saucepan from the sugar syrup, the remainder left in the pan might harden into something that no amount of scrubbing will remove. In that instance, fill the pan with water and bring to a boil. That will soften the leftover syrup, and it will magically melt away with no scrubbing needed.
Notes
For a successful meringue, the egg whites need to be absolutely free of fat to whip. To make sure there is no yolk, I suggest you separate the eggs over a bowl and dump the whites into another bowl (an egg separator can help). Also, wiping the mixing bowl with white vinegar or lemon juice prior to adding the whites helps to ensure there is no fat in the bowl that can inhibit the meringue formation. This step is optional but highly recommended.Cream of tartar is used to stabilize the egg whites as they're being whipped. It isn't strictly necessary, but it is recommended.If desired, you can replace corn syrup with ¾ cup honey (255 grams).You can flavor the marshmallow cream with different extracts, like ¼ teaspoon of almond, mint, or Fiori di Sicilia (an orange vanilla extract). You can even add 1 to 2 tablespoons of whiskey or bourbon for a boozy twist!