Start by concentrating your champagne: Measure 2 cups into a small saucepan, bring to a simmer and reduce until you have 1 cup left. (This will take between 10 - 25 minutes depending on the strength of your simmer.) Remove from heat and cool to room temperature.
Meanwhile, measure flour, baking powder and salt into a medium bowl. Set aside.
While the champagne continues to cool, prep your pans for baking. Grease and flour each of the 3 8-inch round pans, taking care to coat them well. Preheat oven 350 degrees F.
In a large bowl, cream together softened butter and sugar. When nice and fluffy, add sour cream and vanilla extract until well-combined. Add egg whites, beating until fluffy.
Add your dry ingredients and cooled champagne/milk to the wet ingredients in alternating thirds. Scrape sides as needed. Mix until combined.
Divide batter between 3 pans evenly. Bake for 22-25 minutes, until a toothpick comes out clean.
Cool for 5 minutes in the pan before turning out onto a wire rack. Allow to cool completely before frosting.
To make the frosting: Beat together softened butter, powdered sugar, vanilla extract and champagne. To that mixture, add milk—starting with ⅔ cup and adding more as needed up to ¾ of a cup—until it reaches the right consistency.
Frost the bottom layer of the cake, add another layer and coat the top in frosting, and then add the final layer and coat the top and sides. Decorate as desired.
Serve with champagne.