Preheat the oven to 350 degrees F.
Butter and flour a 9-inch cake pan, a 6-inch cake pan, and a 3-inch cake pan.
To make the cake, cream the butter and sugar on medium-high speed in the bowl of an electric mixer fitted with a paddle attachment until light and fluffy, about 5 minutes. On medium speed, add the eggs, 2 at a time, then the sour cream, vanilla, and lemon zest, scraping down the bowl as needed. Mix well. Sift together the flour, cornstarch, salt, and baking soda. With the mixer on low speed, slowly add the flour mixture to the butter mixture and stir just until smooth. Finish mixing by hand to be sure the batter is well mixed. Proportionately divide the batter between the 3 cake pans. Smooth the tops with a spatula, and bake as follows: the 9-inch pan for 45 minutes, the 6-inch pan for 35 to 40 minutes and the 3-inch pan for 30 minutes, or until a toothpick comes out clean in each cake. Cool in the pans to room temperature.
For the frosting:
For the frosting, cream the butter in the bowl of an electric mixer fitted with the paddle attachment. Add the confectioners' sugar, milk, and vanilla extract. Mix on low speed for 1 to 2 minutes, until thickened. Add the food coloring and turn the mixer off. Using a spatula, fully incorporate the color into the frosting.
Frost each cake separately, saving some frosting for decoration. Chill the cakes before stacking.
Place the 9-inch cake on a plate. Center the 6-inch cake on top of the 9-inch cake. Center the 3-inch cake on top of the 6-inch cake.
For the decoration, pipe or spoon the remaining frosting around the base of each cake and around the top edge of the 3-inch cake. Finish decorating with the pieces of candy.
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