In a large bowl, sift together the cake flour, baking powder, and salt.

In a liquid measuring cup, whisk together the buttermilk, vanilla, and lemon extract.

Add the granulated sugar to a large bowl or the bowl of a stand mixer. Rub the lemon zest into the sugar until the sugar is fragrant. Add the butter and beat at medium speed for 3 minutes, or until very light and fluffy.

Add ⅓ of the flour mixture, beating on low speed. Scrape down the sides of the bowl with a rubber spatula and beat in half of the buttermilk mixture, then beat in half of the remaining flour mixture. Scrape the bowl again and add the remaining milk, beating until fully incorporated. Add the remaining flour and beat until ingredients are combined and batter is smooth.

In another bowl, beat the egg whites to stiff peaks.

Gently stir in ¼ of the egg whites into the batter until no streaks remain. Add the remaining egg whites, gently folding them into the batter by cutting the spatula down the middle to the bottom of the bowl, then sweeping it around, up, and over. Continue to fold until the egg whites are incorporated and no streaks remain.

Divide batter between the prepared cake pans.

Bake for 25-30 minutes, until a toothpick inserted into the center comes out clean. Let the cakes cool completely on a wire rack before assembling.

Add a dab of frosting to the middle of your cake plate and set the first layer on top. (This will prevent the cake from sliding while frosting.) Use an offset spatula to spread a thin layer of frosting onto the top. Pipe a line of frosting around the top edge of the cake to create a barrier for the filling.

Spread the lemon curd evenly on the cake, staying inside the frosting barrier.

Flip the second layer of cake over so the bottom is now the top, and place it onto the first layer. Frost the sides and top of the cake as desired.