Combine the butter, sugar, flour, baking powder, and salt in the bowl of a large stand mixer fitted with a paddle attachment. Beat on medium speed until the butter coats the dry ingredients evenly and no large chunks are visible. On low speed, slowly add the eggs, scraping down the sides of the bowl with a spatula to ensure that all of the ingredients are incorporated. Add the vanilla extract and milk, and mix thoroughly. Then, beat the batter on high speed for 15 seconds. Divide the batter into three separate bowls. To create the ombre effect, add yellow gel color to two of the bowls of batter, making one batter a lighter shade of yellow than the other. Do not add gel color to the third bowl of batter. Distribute the batter evenly among three half sheet pans, lined with parchment paper, and bake at 325ºF/163ºC for 15 to 18 minutes. Cool completely.
Combine the buttercream base and butter in the bowl of a stand mixer fitted with a whisk attachment. Whisk together on high speed until smooth. Slowly add the water. Allow each addition of water to fully incorporate, adjusting the speed as necessary. Then, whip the buttercream mixture at high speed for an additional 2 minutes, or until it is fluffy and light. Add the lemon concentrate to taste. Then, add gel color to reach the desired shade of yellow. Set aside.
Assembly
Place a half-sheet frame onto the Lemon Cake layer with no added gel coloring and trim the sides as necessary. Using an offset spatula, spread a thin, even layer of Sizilia Lemon Buttercream. Top with the lightest yellow cake layer. Add another thin layer of buttercream. Top with the remaining darkest yellow cake layer and refrigerate until the buttercream is completely firm. Using a 3 in/7.6 cm round cutter, cut out 20 mini cakes. Using a star tip, pipe a rosette of Sizilia Lemon Buttercream on top of each mini cake.
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