Preheat your oven to 350 degrees F. Grease and line three 8" or 9" round cake pans with parchment paper and set aside.

Place the 8 egg whites in the bowl of a stand mixer fitted with the whisk attachment (or in a large bowl if using a handheld mixer). Mix on high speed until you achieve stiff peaks. Transfer the whipped egg whites to a large bowl and set aside.

Wash the bowl of the stand mixer and dry.

In the same bowl of the stand mixer, cream the butter with the sugar on medium speed with the paddle attachment until light and fluffy.

Meanwhile, in a large bowl, whisk together the flour, baking powder, baking soda and salt. Set aside.

In a large measuring cup, mix together the champagne, buttermilk and vanilla and set aside.

Turn the stand mixer to low and add in the 4 egg yolks, one at a time. Mix until well incorporated.

Alternatively add the dry ingredients and champagne, beginning and ending with the dry ingredients. Mix until everything just comes together.

Stop the mixer and gently fold in the whipped egg whites. Do not over mix. Evenly distribute the batter among the 3 cake pans.

Bake in your preheated oven for 30-40 minutes or until a toothpick inserted in the center comes out clean. Allow the cakes to cool completely before frosting.

In the bowl of a stand mixer fitted with the paddle attachment or in a large bowl if using a handheld mixer, cream the butter on medium speed until light and fluffy. Turn the mixer to low and add in the powdered sugar, about 1/2 cup at a time.

Add the champagne and mix until you achieve the consistency you want. You can add more powdered sugar if it's too runny or more champagne if it's too stiff. Finally, add in the vanilla extract.

Place one cake layer (flat side down) on a cake plate. Top with about 1/2 cup of buttercream. Add sliced fruit on top if desired. Top with another cake layer, then with another layer of frosting and fruit. Put the last cake layer on top, flat side UP (so the top of the cake is smooth).