2 (50 g) egg whites, at room temperature
1 1/4 cups (250 g) granulated sugar
1/2 cup (4 fluid ounces) water
1/8 teaspoon cream of tartar
1/8 teaspoon kosher salt
In the bowl of a stand mixer fitted with the whisk attachment or a large bowl with a handheld mixer, whip the egg whites until they form soft peaks. In a medium saucepan, place the sugar, water, cream of tartar and salt, and whisk together. Cook the mixture, undisturbed, over medium-high heat until it reaches the softball stage (240°F on an instant-read candy thermometer).
Remove the cooked sugar from the heat and allow it cool for five minutes before pouring the cooked sugar mixture carefully down the side of the mixer bowl, with the mixer on low speed (making sure the sugar mixture doesn’t hit the whisk). Increase the mixer to high speed and beat until the mixture thickens considerably, becomes glossy, and mostly stiff peaks begin to form (about 5 minutes).
Place the marshmallow creme in a sealed container and store in the refrigerator. After about 4 days in the refrigerator, the sugar in the mixture will begin to crystallize.