3 cups (24 fluid ounces) Greek-style plain yogurt (at least 2%, or your FroYo will be too icy)
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
First, place the plain yogurt in a large bowl and whisk vigorously until very smooth. Place the bowl in the freezer to chill while you make the marshmallow fluff.
To make the marshmallow fluff, 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 (about 1 minute). 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 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 stiff peaks form (about 5 minutes) and the mixture thickens considerably.
Remove the yogurt from the freezer and whisk again to break up any clumps that have formed. Add about 1/2 of the marshmallow fluff to the chilled yogurt and whisk to combine. Add the remaining marshmallow fluff, and fold it in carefully, taking care not to deflate the fluff. Transfer the yogurt mixture to a freezer-safe container, cover it and place it in the freezer for about 4 hours for soft-serve consistency or overnight for harder, more ice cream-like consistency. If frozen for longer than 12 hours, leave out on the counter for 10 minutes for the frozen yogurt to reach a scoopable consistency.