Fluffy Frosting

Combine sugar, water, and cream of tartar in a small saucepan over medium heat, stirring until the sugar fully dissolves. Continue cooking without stirring until the syrup reaches the soft-ball stage, approximately 240°F (115°C) on a candy thermometer.

While the syrup is cooking, place egg whites in a large, clean mixing bowl and beat with an electric mixer on medium speed until soft peaks form.

Once the sugar syrup reaches the correct temperature, remove it from the heat immediately and allow any violent bubbling to subside for about 30 seconds.

With the mixer running on medium-high speed, carefully pour the hot syrup in a slow, thin, steady stream down the side of the bowl into the beaten egg whites, avoiding the beaters.

Increase the mixer speed to high and continue beating until the frosting becomes very thick, glossy, and holds stiff peaks.

Add the vanilla extract and beat for an additional 30 seconds until fully incorporated and the frosting is brilliantly white.

Immediately spread or pipe the fluffy frosting over the cooled pie or tart, using a spatula or spoon to create decorative swirls and peaks as desired.