Preheat the oven to 350 degrees Fahrenheit and grease three 6-inch cake pans (please see notes), line with parchment paper rounds, then grease the parchment paper.
Whisk the flour, baking powder and salt together in a large mixing bowl.
Using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar together on high speed for about 3 minutes. Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed.
Turn the mixer down to medium-high speed and beat in the 5 egg whites one at a time, letting each egg white fully mix into the batter before adding the next. Beat in the vanilla extract until combined.
Add about 1/3 of the dry ingredients and 1/2 of the champagne and beat on low speed until combined. Add 1/3 more of the dry ingredients and the rest of the Champagne. Beat on low speed until combined then, add the rest of the dry ingredients and beat on low speed until combined.
Add a small amount of food color gel and mix to incorporate.
Divide the batter evenly between the 3 cake pans smoothing the tops with a spatula.
Bake for about 35 minutes or until a toothpick inserted into the center of the cake comes out clean with only a few dried cake crumbs clinging. Start checking for doneness after 28 minutes or so.
Allow the cakes to cool in the pans for 10 minutes then turn out onto a wire rack. The cakes must be completely cool before frosting and assembling.