Cake: Preheat the oven to 180 degrees C. Grease a 15 x 25cm lamington cake tin.
Place all cake ingredients in a mixing bowl then beat them together until smooth. Spoon this mixture into the prepared cake 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.
Chocolate Buttercream: Place the butter, icing sugar and cocoa into a large bowl then mix well until the mixture is of a smooth spreadable.
To Decorate: Cut the cooled cake into rectangles. Spread each cake with some of the buttercream. Place the smarties on the top so that it looks like domino dots and arrange on a serving plate.