Preheat oven to 350F and line a 12-cup muffin tin with paper liners.

Whisk flour, baking powder, and salt in a bowl.

Beat butter and sugar together until pale and fluffy, about 3 minutes.

Add egg whites one at a time, beating well after each addition.

With the mixer on low, add the flour mixture in three additions, alternating with the champagne and sour cream, beginning and ending with flour; mix just until no streaks remain.

Stir in vanilla, then divide the batter evenly among the liners, filling each about two-thirds full.

Bake 16 to 18 minutes, until a toothpick inserted in the center comes out clean and the tops spring back when lightly pressed. Cool completely on a wire rack.

For the buttercream, beat butter until smooth, then add powdered sugar one cup at a time, beating well between additions.

Beat in the 3 tablespoons champagne and a pinch of salt until the buttercream is light and spreadable, about 2 minutes.

Pipe the buttercream onto the cooled cupcakes and finish each with a few gold sugar pearls and a small piece of gold leaf.