Put all the brownie ingredients into a bowl and whisk until light and fluffy. Spoon into a 18x7 inch greased or lined baking tin.
Bake in the preheated oven for 30 minutes. This is how it should look at this stage when cooked.
Wait for it to cool down and then turn it out and cut into rectangles.
For the icing, beat the butter and icing sugar till light and fluffy. Remove two to three spoonfuls of this mixture to use for decorating. Then beat in the cocoa to the remaining mixture till well combined.
Spread the top of each brownie with the chocolate icing. Place the plain icing in a piping bag (or a plastic bag with the corner cut off) and pipe a straight line across the middle of each rectangle. Then place the Smarties® on either side, creating the dominoes.