Three 6-inch round cake pans, 2 large star piping tips, 2 piping bags, 6-inch cake board
For the cake layers: Preheat the oven to 350 degrees F. Spray three 6-inch round cake pans with cooking spray. Dust the bottom and sides of the pans with flour, tapping out the excess. Line the bottom of each with a circle of parchment paper.
Follow the package instructions for the cake mixes, adding all required ingredients. Stir in food coloring until the batter is lemon-yellow color. Divide the batter among the cake pans. Bake until a toothpick inserted in the center of each cake comes out clean, about 24 minutes. Cool the pans on a wire rack for 15 minutes, then unmold the cakes, remove the parchment and cool the cakes completely. Once the cakes are cool, use a serrated knife to slice off any domes from the tops of the cakes to make them flat. Use the knife to split each cake, creating 6 layers of cake total.
For the buttercream: Beat the butter on high speed in a stand mixer fitted with the paddle attachment until smooth, about 2 minutes. Add the confectioners' sugar, salt and vanilla and mix on low to combine. Increase speed gradually to high and beat until smooth. Add 1/2 cup milk and beat until light and fluffy, adding milk a tablespoon at a time to make a smooth buttercream. Transfer about 3/4 cup buttercream to a small bowl and stir in about 1 teaspoon of brown food coloring to make a medium-brown frosting. Set aside. Add 1 1/2 to 2 teaspoons of lemon-yellow food coloring to the remaining buttercream to make a bright-yellow frosting.
Spread 1 to 2 tablespoons of the yellow buttercream over the center of the cake board. Set down the first layer of cake onto the board and press down gently in the center to adhere the cake to the board. Spread a scant 1/2 cup of yellow buttercream over the first layer, spreading it to the edges with a small offset spatula. Top with another cake layer and spread with frosting. Repeat with the remaining cake layers, leaving the top of the final layer bare. (If desired, stick 3 skewers or dowels into the cake at this point to reinforce it, but be sure to remove the skewers before serving.)
Using a small or medium offset spatula, frost the entire cake with a thin layer (crumb coat) of buttercream; it's okay if some spots of cake show through. Refrigerate the cake to firm the buttercream.
For the leaves: Put the candy melts in a microwave-safe bowl. Add the vegetable oil. Microwave on 50-percent power in 30-second intervals, stirring in between intervals, until melted. Stir until very smooth. Line a baking sheet with a silicone mat. Using a soup spoon, scoop up a spoonful of the mixture and drop it onto the silicone mat. Use the back of the spoon to smooth it into a leaf shape, tapering it at one end. Repeat, making a total of 6 to 10 leaves, some larger and some smaller. Use the side of an offset spatula to score the leaves into pointed teardrop shapes. Allow the leaves to cool at room temperature until set, 10 to 15 minutes. Carefully remove the leaves from the mat and remove excess candy so you have teardrop-shaped leaves.
Put the remaining yellow buttercream in a piping bag fitted with a large star tip. Starting at the bottom, pipe a ring of stars around the cake. Continue piping stars around the cake, staggering the stars as you move up the sides of the cake so all the cake is covered. Pipe all the way to the top of the cake, then pipe 2 rings of yellow stars over the top, leaving a 3- to 4-inch circle empty at the top. Put the brown buttercream into a piping bag fitted with a large star tip. Pipe brown stars into the empty circle. Stick the leaves into the brown stars on top of the cake, putting the larger leaves in the center and the smaller leaves around the outside.