Preheat the oven to 180 C. Place all of the cake ingredients in a mixing bowl and beat them together until smooth. Spoon this mixture into a greased 25x15cm tin. Bake for 30 minutes, or until the surface of the cake springs back when gently pressed with a finger. Turn out onto a wire rack and allow to cool.
For the chocolate buttercream, place the butter, icing sugar and cocoa powder into a large bowl and mix well until the mixture is of a smooth spreadable consistency.
Cut the cooled cake into squares. Spread each cake with some of the buttercream. Place the coloured sweets on the top so that it looks like domino dots, and arrange on a serving plate.