Tender and moist champagne cake layers with a raspberry and peach puree and champagne buttercream.
Preheat your oven to 325 degrees F. Spray three 8-inch or four 6-inch round cake pans with nonstick spray, line the bottom with parchment paper and spray again. Set aside.
In a medium sized sauce pan, simmer the 3 cups of Martinelli's or champagne over medium heat. You'll reduce the drink to 1 1/2 cups total. 1 cup is for the cake and 1/2 cup is for the buttercream. Set aside.
In a medium sized mixing bowl, whisk together the cake flour, baking powder and salt. Set aside.
In the bowl of an electric mixer, fitted with the paddle attachment, cream the butter and the sugar for two minutes on medium-high speed.
Scrape down the sides of the bowl and begin to add the egg whites on medium speed, scraping down the sides and bottom of the bowl between addition.
Add the sour cream and beat until incorporated, about one minute.
With the mixer on low speed, add 1/3 of the flour mixture, followed by 1/2 cup of the Martinelli's reduction. Repeat the additions, 1/3 cup flour mixture, 1/2 cup Martinelli's and the final 1/3 cup flour mixture. Add vanilla. Only mix until the flour mixture is incorporated, about 30 seconds. Scrape down the sides and bottom of the bowl and mix again for another 20 to 30 seconds on low.
Pour about 18 ounces of cake batter into each of the three cake pans and bake for 28-30 minutes - until a toothpick inserted into the center of the cake comes out with a few moist crumbs on it. Let cool in the pans for about 15 minutes and then invert onto a wire rack to cool completely. (Tip: spray your wire cooling racks with nonstick spray).
In a medium sized sauce pan, combine the raspberries, peaches, and sugar and simmer for about 5 minutes. Once the fruit is tender and the juices have started to release, puree the mixture in a blender or food processor.
Strain the fruit puree and pour the strained fruit puree back into the sauce pan. Add the corn starch and simmer for about 5 more minutes. The mixture should get thicker.
Cool the puree completely and refrigerate to set a little more. The texture should ultimately be like a jelly. Best to make a day or more before you assemble the cake.
In the bowl of an electric mixer fitted with the paddle attachment, cream the butter on medium-high speed for about 2-3 minutes.
With the mixer on low speed, gradually add the sifted powdered sugar, followed by the heavy cream, Martinelli's reduction and pinch of salt. Turn the mixer to medium-high speed and beat for 3-5 minutes.
Before applying your final coat of frosting, be sure to mix the buttercream by hand with a wooden spoon or large spatula to push out the air pockets.
Spread a small dollop of frosting on a cake board or cake plate. Place the first cake layer, top side up, in the center of the cake board.
Spread about 1/2 cup of the frosting over the cake layer and then pipe a rim of frosting around the edge of the cake. This will act as a barrier to the filling.
Spread half of the fruit filling over the cake.
Repeat these steps for the next cake layer and then add the final cake layer, top side down, on the very top.
Apply a thin layer of frosting around the entire cake. This is the crumb coat. You'll want to then freeze the crumb coat for about 10 minutes to set this layer of frosting. Once the crumb coat is set, you can use the remaining frosting to cover the cake.