1) Preheat the oven to 180°C/gas mark 4. Butter and flour a 23cm cake tin, 15cm cake tin, and a 7cm cake tin.
2) Cream the butter and caster 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, cornflour, salt, and bicarbonate of 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.
3) Proportionately divide the batter between the 3 cake pans. Smooth the tops with a spatula, and bake as follows: the 23cm tin for 45 minutes, the 15cm tin for 35 to 40 minutes and the 7cm tin for 30 minutes, or until a toothpick comes out clean in each cake. Cool in the pans to room temperature.
For the frosting:
1) Cream the butter in the bowl of an electric mixer fitted with the paddle attachment. Add the icing sugar, milk, and vanilla essence. 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 colour into the icing.
2) Ice each cake separately, saving some for decoration. Chill the cakes before stacking.
3) Place the 23cm cake on a plate. Center the 15cm cake on top of the 23cm cake. Centre the 7cm cake on top of the 15cm cake.
4) For the decoration, pipe or spoon the remaining frosting around the base of each cake and around the top edge of the 7cm cake. Finish decorating with the pieces of candy.