For the Cakes (makes 3 8" cakes):
For the Buttercream Frosting:
Preheat oven to 350 degrees Fahrenheit. Butter 3 8 x 2" cake pans, line with parchment rounds and dust with flour, tapping to remove the excess.
Sift together flour, baking powder and salt in a medium bowl. Set aside. Combine the buttermilk, scraped vanilla bean seeds and vanilla extract together in a small bowl and set aside.
In a stand mixer fitted with the paddle attachment, beat the butter and sugar on medium speed until light and fluffy, about 5 minutes. Lower the speed and gradually add in the egg whites until they are fully incorporated.
With the mixer on low speed, alternately add the dry flour mixture and the vanilla-buttermilk mixture to the creamed butter-and-egg mixture until just combined.
To make the frosting, make sure your stand mixer bowl is very clean and it has been fitted with the whisk attachment. Add the egg whites to the bowl and whisk on medium speed until foamy. Gradually add 6 tablespoons of the sugar and beat on high speed until medium firm peaks form.
Divide the batter evenly between the 3 cake pans and smooth the tops with a spatula or palette knife. Bake for 30-35 minutes, rotating once to ensure even baking, until the center is firm but sponge-y to the touch or a cake tester inserted in the center of each cake comes out clean. Allow cakes to cool on a wire rack for 10 minutes in the pans and then turn them out on the rack to cool further until assembly.
Combine the remaining 3/4 cups plus 2 tablespoons of sugar with the corn syrup in a medium saucepan. Bring the mixture to a boil over medium-heat, stirring to dissolve the sugar. Once the mixture comes to a rolling boil and there are no pockets of un-dissolved sugar, remove the pan from the heat. Promptly and gently pour the hot syrup down the sides of the egg white-mixer in a steady stream with the mixer whisking at medium-high speed. Lower the speed to medium and let the whisk work it's magic until the bottom of the bowl feels lukewarm to the touch.
Slowly add in the butter, roughly one tablespoon at a time, until it has all been incorporated. Add the vanilla extract, salt and any food coloring you might choose (we did plain white, so no food coloring for us) and continue beating the frosting until thickened and smooth. This will take about 5 minutes.
To assemble the cake, place one cake on a cake platter or plate and frost with about 1 cup of frosting just on the top of the cake. Add the second layer of cake and frost in the same way. Add the final layer of cake with the cake facing down (so the flat baked bottom of the cake is at the top). Spread a thin layer of frosting all over the top and sides of the cake, scraping off any excess, this is the crumb coat. Put the cake in the fridge to cool and firm up for about an hour. Then, use the rest of the frosting to coat the entire top and sides of the cake in a thick layer. Decorate as desired.