Preheat your oven to 375°F (190°C) and line a muffin pan with festive cupcake liners.

In a large mixing bowl, cream the softened unsalted butter and granulated sugar until light and fluffy.

Add the egg whites, mixing well after each addition until fully incorporated.

Sift together the all-purpose flour, baking soda, salt, and baking powder in a separate bowl.

Whisk together the vanilla extract, sour cream, and milk until smooth in another bowl.

Gradually combine the dry and wet ingredients into the creamed butter and sugar, alternating and mixing just until combined.

Gently fold in the Christmas-colored Funfetti sprinkles using a spatula.

Fill each cupcake liner about two-thirds full with batter.

Bake for approximately 18 minutes, or until a toothpick comes out clean.

Cool the cupcakes in the pan for about 5 minutes before transferring them to a wire rack.

To make the buttercream, beat the unsalted butter until fluffy, then gradually add powdered sugar, mixing well.

Once fully mixed, incorporate the vanilla extract and whipping cream until creamy and slightly stiff.

Frost each cooled cupcake generously with the buttercream and top with additional sprinkles.