Tender and flavorful vanilla bean cake layers with a coffee, rum soak, mascarpone custard and coffee buttercream.
Preheat your oven to 325 degrees F. Spray three 8-inch or four 6-inch round cake pans with non-stick spray and then line with parchment. Spray again. 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 3 to 5 minutes on medium-high speed. The mixture should be light and fluffy.
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 vanilla extract and beat until incorporated.
With the mixer on low speed, add 1/3 of the flour mixture, followed by 1/2 cup of the buttermilk. Repeat the additions, 1/3 cup flour mixture, 1/2 cup buttermilk and the final 1/3 cup flour mixture. 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. Fold in the sour cream.
Pour about 16 ounces of cake batter into each of the three 8-inch cake pans or 12 ounces in each of the 6-inch pans and bake for 25 to 28 minutes, or 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).
Combine the coffee and rum extract and stir.
In the bowl of an electric mixer, fitted with the paddle attachment, beat the butter for about 2 minutes, until light and fluffy and pale in color.
With the mixer on slow, gradually add the powdered sugar, followed by the heavy cream, coffee emulsion, and pinch of salt.
Once all of the ingredients are combined, turn the mixer to medium-high and beat for about 3 to 5 minutes. Scrape down the sides of the bowl and beat for a couple minutes more.
Before using the buttercream, make sure to beat it by hand with a wooden spoon or large spatula to help push out the air pockets.
In a stainless steal bowl (preferrably chilled in thre freezer for 10 minutes), beat the cream until stiff peaks form. Set aside.
In a double boiler, with about an inch of water in the bottom pot, combine the sugar and egg yolks. Stir constantly while the water boils underneath, until the mixture is light yellow in color, about 8 to 10 minutes.
Once the egg mixture is cooked, add it to the mascarpone cheese in a mixing bowl, and beat together until combined. Add the vanilla and stir to combine.
Fold the mascarpone mixture into the whipped cream. Cover and chilled until ready to use.
Trim the tops of the cake layers slightly, so the crumb is exposed and can soak up the coffee soak.
With a pastry brush, cover the cake layers with the coffee soak. I recommend going over each layer twice with the soak.
After you've done the soak, place the first cake layer top side up, in the center of the cake board. Pipe a rim of buttercream around the edge of the cake to create a barrier for the custard.
Spread about 1 cup of the custard across the cake layer.
Place the second cake layer on top of the buttercream and repeat the two previous steps.
Place the final cake layer top side down on the filling.
Apply a thin layer of buttercream around the entire cake. Freeze the cake for 10 minutes. This is your crumb coat.
Once the crumb coat is set, continue frosting the cake with the remaining buttercream.