Preheat oven to 350F. Prepare three 6" cake pans by greasing and lining with parchment paper. Set aside

In a medium-sized bowl, whisk together flour, baking powder, and salt until homogeneous. Set aside.

Using a hand mixer (or a stand mixer with the paddle attachment), beat the butter and oil together until smooth. Add the sugars and beat on medium-high speed until light and fluffy, this will take about 3 minutes.

Next, add the egg whites, one at a time, making sure each is fully incorporated into the batter before adding the next. After the egg whites are added, mix in the vanilla.

Next, adding 1/3 of the flour mixture and mix until just combined. Then add 1/2 of the champagne and mix to combine. Add another 1/3 of the flour mixture, mixing until combined. Add the last half of the champagne and mix. Finally, add the last 1/3 of the flour mixture. Mix until everything is just combined and no streaks of flour remain. Use a spatula to scrape the sides and bottom of the bowl, folding the batter a bit, making sure everything is mixed evenly.

Distribute batter evenly amongst the pans and bake in the preheated oven for 35-40 minutes or until a toothpick inserted into the center comes out clean.

Allow to cool for 10 minutes before taking the cakes out of the pans and then allowing the cakes to cool completely.

Add the butter, powdered sugar, and salt to a bowl. Beat on low speed to combine. Once the powdered sugar is combined, increase the speed to medium-high and beat until fluffy.

Add the vanilla and champagne and beat until fully incorporated.

If the buttercream is a bit too thin, add 1/2 cup of powdered sugar at a time until you reach the consistency you like. If the buttercream is too thick, add 1 tablespoon more of champagne at a time until you reach the consistency you like. Wait until the cakes are completely cooled before leveling and frosting. Enjoy!