February 1, 2019
For the cupcakes: Preheat oven to 350 degrees and line 12 muffin cups with paper liners. In a large bowl, beat together the butter and sugar until light and fluffy. Add the egg whites and vanilla, and beat until smooth, 1 to 2 minutes. In a bowl, whisk together the cake flour, baking powder and salt. Then, stir the flour mixture into the butter mixture until combined. Add the sour cream and heavy cream, and beat until smooth and thick. Evenly scoop the batter into the prepared muffin cups and bake until a cake tester inserted into the center of a cupcake comes out clean, 18 to 20 minutes. Let cool completely in the pan on a wire rack.
For the frosting: In a large heatproof bowl, whisk the egg whites until soft peaks form. Meanwhile, in a small saucepan, stir together the sugar, water and corn syrup. Heat the sugar mixture until it reaches 230 degrees on an instant-read thermometer. With your mixer on, carefully pour the hot sugar mixture into the egg whites in a very slow stream, until it is completely incorporated. Beat the mixture until it has completely cooled and stiff, glossy peaks have formed, about 10 minutes. Mix in the softened butter, 1 tablespoon at a time, until all of the butter has been added. Continue beating until the mixture comes together and becomes smooth and fluffy. Add the vanilla and salt, and beat to combine. Frost the cupcakes, then decorate with white sprinkles.