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.