Lemon cake layers baked on a shortbread crust, with lemon curd filling and lemon buttercream.
Preheat the oven to 325 degrees F. Spray three 8-inch or four 6-inch round cake pans with nonstick spray, line each pan with parchment and spray again. Set aside.
Combine the sugar and butter in the bowl of a stand mixer fitted with the paddle attachment. Beat for 1 to 2 minutes until creamy. Stir in the salt and flour. If the mixture is too crumbly, you can add another tablespoon of butter.
Evenly divide the shortbread cookie crumbs into each cake pan (about 8 ounces in the 8-inch pans or 6 ounces in the 6-inch pans). Using the back of a spoon, press the shortbread cookie crumbs into an even layer on the bottom of the pan.
Bake for about 6 minutes. Will be light in color. Let cool while you make the cake batter.
Preheat your oven to 325 degrees F.
In a medium size bowl, lightly whisk the egg whites and add the lemon extract. Set aside.
In another medium size bowl, combine the dry ingredients except the sugar and pudding mix. Whisk to combine. Set aside.
In a bowl of an electric mixer fitted with a paddle attachment, cream the butter, sugar and pudding mix together on medium speed, until light and fluffy, about 2 to 3 minutes.
Scrape down the sides of the bowl and begin to add the egg mixture in 3 separate batches, mixing until well combined, about two minutes. Scrape down the sides and bottom of the bowl to ensure all the batter has been incorporated.
With the mixer on low speed, alternately add the flour mixture and the buttermilk, starting and ending with the flour mixture. Mix until combined. Fold in food coloring, if using.
Divide batter evenly among your pans, right on top of the shortbread crust, about 16 oz of batter in each 8-inch pan or 12 oz in each 6-inch pan.
Bake the cakes (on the shortbread crust) until a toothpick comes out with a few crumbs inserted into the center, about 25 to 30 minutes. Let cool on wire racks for 10 minutes before inverting them onto cooling racks to cool completely. The cake will stick to the shortbread crust and flip out of the pan together.
Whisk the sugar, lemon zest, and lemon juice in a medium saucepan. Whisk in the whole eggs and yolks in a small bowl and then whisk them into the lemon mixture.
Cook the mixture over medium-low heat, whisking constantly, until it's thick like pudding, about 6 to 8 minutes. Remove from the heat and whisk in the butter, a few pieces at a time until incorporated.
Strain the mixture through a fine-mesh sieve into a small bowl, pushing it through with a rubber spatula. Press plastic wrap directly onto the surface to prevent a skin from forming. Refrigerate until completely set, at least 4 hours and up to 5 days.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter until light and fluffy, about 2 minutes.
Turn the mixer to low and gradually add the powdered sugar, followed by the cream, salt and lemon extract. Mix until combined. Add yellow food coloring, if using. Mix until incorporated completely. Turn the mixer to medium-high and beat the frosting for about 5 minutes.
Use wooden spoon to beat the frosting by hand for about 2 minutes to push out any air pockets.
Spread a small dollop of frosting onto a cake board. Place the first cake layer, top side up, on the board.
Spread a thin layer of frosting over the cake layer and pipe a rim of frosting around the edge of the cake to create a barrier to the filling. Spread about 1/4 cup of the curd in the center of the cake, carefully spreading it to the edge of the piping.
Place a second cake layer on top of the piped frosting and filling and repeat step 2. Place the final cake layer, top side up, on the second layer of filling.
Lightly coat the cake with a thin layer of frosting to lock in the crumbs. Freeze the cake for about 10 to 15 minutes to set the frosting.
Finish frosting and decorating the cake with the remaining frosting. Serve a spoonful of the remaining lemon curd with each slice of cake.
The cake can be stored in the freezer for about a week, in an airtight container. Give the cake about 8 hours to return to room temperature before serving.