Cook champagne over medium heat until 1 cup remains, then cool.

Preheat oven to 350°F and prepare three 8-inch cake pans with parchment paper and grease the sides.

Cream butter and sugar until light and fluffy, then add sour cream and vanilla, mixing until combined.

Add egg whites in two batches, mixing well and scraping sides as needed.

Combine dry ingredients in a separate bowl. Mix milk with cooled champagne.

Alternate adding dry ingredients and champagne mixture to the batter, starting and ending with dry ingredients.

Divide batter evenly into pans and bake for 23-27 minutes or until a toothpick comes out clean.

Cool cakes in pans for 2-3 minutes, then transfer to racks to cool completely.