Then it's time for the cake layers! Preheat the oven to 350°F/175°C and line and grease three, 8-inch cake pans or four, 7-inch cake pans with parchment rounds and non-stick spray (I used 7-inch cake pans).
Add 3 cups sugar and 2 Tbsp lemon zest into a large bowl or the bowl of a stand mixer. Use your fingertips to massage the zest into the sugar to help release the oil in the zest. You should be able to smell the oils being incorporated into the sugar.
Add 3 cups cake flour, 2 1/2 tsp baking powder, and 1 tsp salt into the bowl with the sugar and whisk together until combined.
Then mix in 1 cup (2 sticks) of unsalted butter with a paddle attachment or hand mixer on a low speed. Continue to mix until no large chunks of butter remain, and the mixture looks like moist sand.
Mix in 1 cup of egg whites on a medium speed until incorporated. The batter should be quite thick at this point.
Then add in 1 cup sour cream, 1/2 cup lavender milk (made in the recipe above), 2 Tbsp vegetable oil, and 1 tsp vanilla extract. Mix on a low speed until incorporated.
Scrape down the sides and bottom of the bowl with a rubber spatula, then beat on a medium speed for about 30 seconds to make sure everything is properly mixed together. This cake recipe uses the reverse creaming method, so this step is important to help give the cake layers a lighter texture.
Pour half of the cake batter (about 900g) into a separate bowl. Use a small drop of gel food coloring to color one bowl of batter purple and the other bowl yellow.
Alternate spoonfuls of both colors of batter into the prepared pans. Use a butter knife or a small offset spatula to swirl the batter together. Bake for 33-36 minutes or until a toothpick comes out with a few moist crumbs. Let the cake layers cool in the pans for 10 minutes, then run a small offset spatula around the perimeter of the pan to separate the cake from the pan.
Flip the cake layers onto a wire rack to cool. Use a serrated knife to level the top of the layers once they're fully cooled.
If you make these cake layers in advance and freeze them, let them thaw for about 20 minutes before assembling your cake. The cake layers should still be cold to the touch, which will make it easier to assemble your cake.