• Preheat oven to 325° F. Spray three 6-inch cake pans with cooking spray and line the bottoms with parchment paper.
• In the bowl of a stand mixer fitted with a whisk attachment, add the dry cake mix, flour, sugar, and salt. Mix until combined.
• Switch to the paddle attachment and add milk, lemon juice, sour cream, vegetable oil, vanilla, eggs & lemon zest. Mix on low-medium speed just until combined but do not overmix.
• Pour 1/3 of the batter into each cake pan and bake for about 25 minutes or until the middle is set. Insert a toothpick in the middle of the cake to test.
• While cakes are baking, prepare Lemon Curd and Swiss Meringue Buttercream.

Assembly:

• If the cake layers do not bake evenly, you can use a bread knife to trim the top off and make it flat which is easier for decorating. If you froze the cake layers, you will need to let the cake layers thaw just a bit before trimming.
• Start with the frozen/semi-thawed cake layers, and place one layer on a cake board. You can use a spoonful of buttercream as “glue” between the cake layer and cake board if you like.
• Pipe a circle of buttercream around the perimeter of the cake layer to create a barrier; add a thin layer of Swiss Meringue Buttercream then spread a generous amount of lemon curd over that.
• Add your second cake layer and repeat with the Swiss Meringue Buttercream/lemon curd, then frost the outside with the Swiss Meringue Buttercream.