1Preheat oven to moderate, 180 C. Lightly grease and line 3 x 15-cm square cake pans. Prepare cake mixtures adding 1/3 cup cocoa with sifted flour and extra ¼ cup milk.
2Spoon batter evenly among 3 pans. Cook 25 to 30 mins until cooked when tested. Cool in pans 5 mins. Transfer to a wire rack to cool completely.
3Add extra 1/3 cup cocoa with sifted icing sugar and 1 tbsp extra milk to basic butter cream to make a dark-brown icing.
4Trim tops of cakes to level. Spread top and sides of each cake with butter cream, smoothing with a palette knife.
5Stack cakes on top of each other, icing side up, with centre layer at a 45-degree angle to bottom cake.
6Separate Smarties into colour groups. Using 9 Smarties of each colour, make 9 coloured squares on top of each cake, alternating colours. Continue pattern around sides of cakes and fill in any corner gaps. Cut cake into squares to serve.
Photo: Bauer/Rob Shaw