In a small saucepan set over medium heat add the granulated sugar, corn syrup, water, and pinch of salt. Boil, stirring occasionally until the mixture reaches 240°F on a candy thermometer. (It will take around 8 minutes or so to reach 240°F… you will need to use a candy thermometer for this though)!
While the sugar mixture is boiling add the egg whites and cream of tartar to a stand mixer bowl fitted with a whisk attachment. Whip on medium-high speed until soft peaks form.
Once the sugar reaches 240°F remove from heat, and turn the mixer speed to low (that has the egg whites in it) and carefully and slowly drizzle the syrup mixture down the side of the mixer bowl and into the egg whites. Once all of the syrup has been added turn the mixer to medium-high speed and whip for 7-10 minutes or until stiff glossy peaks form. During the last minute of beating, add the vanilla extract.
Notes
This recipe makes roughly 3 cups worth of marshmallow cream (creme), about 310 grams. Store homemade marshmallow cream in an airtight container at room temperature for two weeks, in the fridge for a month, or in the freezer for up to three months. If you store it in the fridge, let it come to room temperature before using it. If you store it in the freezer, allow it to thaw to room temp, then re-whip it.