FOR THE PINK CHAMPAGNE CAKE LAYERS:
Preheat oven to 350° F (177°C). Prepare three 7-inch round cake pans with parchment rounds, and grease the sides. Set aside.
In a large bowl, sift cake flour, baking powder and salt. Whisk to combine, set aside.
In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and sugar on medium speed until light and fluffy, 4-6 minutes. Scrape down the sides and bottom of the bowl.
Add egg whites one at a time, mixing after each addition until just combined. Add vanilla and mix until combined.
Alternately add dry ingredients in thirds and pink champagne in 2 additions to the batter, beginning and ending with the dry ingredients. Beating each addition just until combined. Tint batter with a few drops of pink gel food colouring, if desired.
Divide batter equally between the three cake pans, about 2 cups per pan. Bake for 30 minutes or until a toothpick inserted into the center comes out clean and cake springs back when lightly touched.
Remove cakes from oven and allow to cool in pans for about 10 minutes, then turn onto cooling racks to cool completely before frosting.
FOR THE CHAMPAGNE BUTTERCREAM FILLING:
In a large bowl with a hand-held mixer, beat the butter, shortening and salt on medium speed until light and fluffy (about 5-6 minutes).
Turn the mixer to low, and gradually add half of the sugar one cup at a time. Once incorporated, add champagne or sparkling wine and beat until combined. Mixture will look curdled. That’s normal.
Add the remaining powdered sugar one cup at a time and mix on low until combined.
Once the sugar is combined, increase mixer speed to high and whip until light and fluffy, about 5-7 minutes.
