Preheat the oven to 325 degrees Fahrenheit, then prep three six-inch round cake pans with a swipe of shortening and dusting of flour to prevent sticking.

In a large bowl, whisk together the buttermilk, sour cream, eggs, oil, vanilla, and lemon zest and juice until thoroughly combined. Sift in the flour and cake mix and whisk until just combined. Do not overmix.

Divide the batter in half in two bowls, then gently fold the raspberry emulsion and gel coloring into one bowl. Split the batter evenly among the prepared pans, alternating between scoops of lemon and raspberry cake batter.

In a medium saucepan over medium heat, whisk together the eggs, sugar, lemon juice, and zest until the mixture thickens, three to five minutes. Remove the curd from the heat and add the butter, one tablespoon at a time. Strain the curd to remove the zest. Transfer to a glass dish, cover, and refrigerate until thickened and cooled completely.

In the bowl of a stand mixer fitted with a paddle attachment, beat the butter on medium speed until it’s light and fluffy.

Add the heavy cream, vanilla, and salt and beat again until combined, then scrape down the sides with a spatula.

With the mixer on low speed, add the powdered sugar about half a cup at a time. The buttercream will be thick. Add a little more heavy cream, one tablespoon at a time, to thin it out if needed, and then flip the mixer to high speed and beat until the buttercream is lighter in color and texture, about two minutes.

Divide the buttercream between two bowls Add the lemon extract and yellow gel coloring to one bowl and the raspberry emulsion and red gel coloring to the other. Fill one piping bag with lemon buttercream and one with raspberry buttercream.

Place a cardboard cake round on a cake turntable. Dot a little bit of buttercream in the center of the round and spread it out with an angled icing spatula.

Place the first chilled cake layer in the center. Add a thin layer of either lemon or raspberry buttercream. Pipe a buttercream dam of the same flavor around the outside rim of the frosted cake layer, then add about cup of the same flavor filling to match the buttercream.

Place the next cake layer on top, then repeat, alternating flavors each time and making sure to line the layers up evenly.

After adding the top cake layer, crumb coat the cake (use the lemon on the top half and the raspberry on the bottom) by spreading a thin layer of buttercream around the entire cake. Freeze for two minutes to set the buttercream.

Using a wooden spoon, beat out any air bubbles in the remaining buttercreams. Finish frosting the cake as desired or follow the decorating instructions below.

Spread a thick final layer of lemon buttercream on the top half of the cake and raspberry along the bottom. Use a wavy cake scraper to make the decorative pattern on the sides and use an angled icing spatula on top to flatten and clean up the edges.

Fill a piping bag fitted with a Wilton 6B tip with the remaining buttercream, then pipe dollops around the top rim of the cake in a crown. Place a lemon slice between each dollop and a raspberry on top of each one.