To make cake, preheat oven to 350 degrees with 1 rack one-third from the bottom of the oven and a second rack two-thirds from the bottom. Line three 9-inch cake pans with parchment paper rounds and spray with Baker's Joy or butter and flour.
In a medium bowl, combine flour, baking soda, salt, and baking powder.
Using an electric mixer, beat butter and sugar on medium speed with a paddle attachment until light and fluffy, about 2 minutes.
Scrape down sides of bowl and add egg whites and vanilla. Beat on medium speed for 1 minute.
Add 1/3 of flour mixture and beat on medium speed until incorporated.
Add half of buttermilk and beat on medium until mixed in.
Scrape down sides of bowl and repeat adding flour mixture and buttermilk, finishing with remaining 1/3 of flour. Batter will be thick and glossy.
Pour batter evenly into prepared pans. Set two cake pans on 1 oven rack and the third pan on the second rack, staggering them so cakes are not directly over each other.
Bake 25 to 35 minutes, until the tops are flat and browned.
Cool cakes in pans for 10 minutes. Invert onto racks to cool completely. (Cakes can be wrapped in plastic wrap and frozen for up to 3 weeks.)
For Buttercream, use an electric mixer with a whisk attachment to beat the eggs and egg yolks at high speed for 5 minutes.
Combine water and sugar in a medium saucepan and simmer until it reaches soft ball stage, between 234 and 240 degrees on a candy thermometer.
Quickly transfer the mixture to a heatproof liquid measuring cup.
In a very slow stream, add this liquid to the egg mixture while the mixer is on low speed.
Increase speed to medium and beat for about 7 minutes, until mixture has cooled. Bowl should just barely feel warm.
Add butter a half stick at a time and beat on medium speed for 20 seconds after each addition.
After all butter has been added continue to beat on medium speed for 3 minutes, until thickened slightly.
Stir in salt and whiskey.
Spread a thick layer of frosting on top of one cake layer. Add second layer and top it with a thick layer of frosting. Add the top layer and frost the top and sides.