For the cake: Preheat the oven to 350 degrees F. Grease two 8-by-3-inch round cake pans with butter.
In a bowl, whisk the egg whites, 1/4 cup of the buttermilk, lemon zest, lemon juice and vanilla until combined. Set aside.
Combine the flour, granulated sugar, baking powder and salt in a stand mixer fitted with the whisk attachment. Mix on low speed for 1 minute. Add the butter and the remaining 1 cup buttermilk. Mix on low speed until the dry ingredients are moistened, then beat on medium speed for 2 minutes. Add one-third of the egg white mixture, beat on low speed until blended and then on medium speed for 20 seconds. Scrape down the bowl and repeat two more times with the remaining egg white mixture.
Divide the batter between the prepared cake pans. Bake for 30 to 40 minutes, until an inserted toothpick comes out clean. Cool the cake in the pans on a wire rack for 10 minutes. Invert the cakes onto the rack and let cool completely.
For the buttercream: Combine the egg whites and powdered sugar in a clean mixer bowl. Using the paddle attachment, mix on medium speed for 10 minutes. Reduce the speed to low and start adding the butter, 1 tablespoon at a time. Once all of the butter is incorporated, beat on medium speed for 10 minutes, until the buttercream comes together.
Puree the raspberries with the granulated sugar until liquid. Press through a fine-mesh sieve. Mix the raspberry juice into the buttercream on low speed until incorporated, then beat for another 2 minutes until smooth.
To assemble the cake: Add buttercream to each cake layer, then stack the layers. Ice the top and sides of the stacked cake with the remaining buttercream.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.