For the cake: Preheat the oven to 375 degrees F. Spray two 9-inch round cake pans with nonstick baking spray.
Sift the cake flour, baking powder and salt into a bowl, then whisk to combine. Whisk the buttermilk and vanilla bean paste together in a liquid measuring cup until smooth. Combine the butter and sugar in the bowl of a stand mixer and beat on medium-high speed until creamy, about 2 minutes. Add the eggs to the mixer; reduce the speed to low and alternately add the flour mixture and buttermilk mixture until just incorporated. Add the blueberries and fold gently with a rubber spatula until just combined.
Divide the batter between the prepared cake pans and smooth the tops. Bake the cakes until golden on top and a toothpick inserted in the middle of each comes out clean, 25 to 30 minutes. Transfer the cake pans to a cooling rack, let cool for 10 minutes, and then invert onto the rack and unmold the cakes from the pans. Flip the cakes right-side up and let cool completely.
For the filling: Place a metal bowl on top of a saucepan filled with 1 inch of water so the bowl fits snuggly inside the pan without touching the water. Remove the bowl from the pan and bring the water to a boil. Combine the sugar, egg yolks, lemon zest and juice in the bowl and whisk until smooth. Reduce the heat to medium-low, set the bowl over the simmering water and cook, stirring constantly, until thickened, about 8 minutes. Remove the bowl from the heat and slowly whisk the butter, one piece at a time, into the curd until melted and smooth. Let the curd cool to room temperature, then refrigerate for at least 2 hours. (Makes approximately 1 cup.)
For the buttercream: Combine the confectioners' sugar, egg whites and vanilla in the bowl of a stand mixer fitted with the paddle attachment. Beat on low speed until all of the sugar is absorbed, then increase the speed to high and beat until light and fluffy, about 8 minutes. Reduce the mixer speed to medium-low and add the butter, 1 tablespoon at a time, waiting until each tablespoon is absorbed before adding the next. Beat until smooth; the mixture will look curdled at first but then will come back together once all the butter is added. Scrape down the sides of the bowl, then beat on high speed until fluffy and smooth, about 2 minutes.
To assemble: Shave the cake layers down to a level surface with a serrated knife, then cut each in half to torte, using a turntable if available. Divide the filling into thirds and spread liberally between each cake layer. Ice the cake with a thin layer of the buttercream to crumb coat, then chill for 15 minutes. Frost to your liking with the remaining buttercream, and enjoy!
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.