Preheat the oven to 350°F and grease and line three 8-inch round cake pans with parchment paper.
In a medium bowl, whisk together the cake flour, baking powder, baking soda, and salt.
In a large mixing bowl, beat 3/4 cup softened butter and the granulated sugar on medium speed for 3 to 4 minutes until light and fluffy.
Add the egg whites one at a time, mixing well after each addition.
Mix in 1 tsp vanilla bean paste and 1/2 tsp almond extract until combined.
In a separate bowl, whisk together 1 cup pink champagne and the sour cream until smooth, then stir in 1/8 tsp pink gel food coloring.
Add one-third of the dry ingredients to the butter mixture and mix on low just to combine.
Add half of the champagne-sour cream mixture and mix on low until incorporated.
Repeat with another third of the dry ingredients, then the remaining champagne mixture, ending with the last third of the dry ingredients, mixing just until the batter is smooth.
Divide the batter evenly among the three prepared pans and smooth the tops.
Bake for 22 to 26 minutes, or until a toothpick inserted in the center comes out clean.
Cool the cakes in the pans for 10 minutes, then turn out onto wire racks to cool completely.
For the buttercream, beat 1 1/2 cups softened butter in a large bowl on medium speed until smooth and creamy.
Gradually add the powdered sugar, about 1 cup at a time, mixing on low speed after each addition until combined.
Add 6 tbsp pink champagne, 1 tsp vanilla bean paste, and 1/8 tsp salt, then increase the speed to medium-high and beat for 2 to 3 minutes until light and fluffy.
Beat in 1/8 tsp pink gel food coloring until the frosting is evenly pink.
Once the cake layers are completely cool, level the tops if needed.
Place one layer on a serving plate and spread a generous layer of buttercream over the top.
Add the second layer, spread more buttercream, then top with the third layer.
Frost the top and sides of the cake with the remaining buttercream, smoothing or swirling as desired.
Chill the assembled cake for 20 minutes before slicing for cleaner slices.