Grease and line a tray tin 13 x 9 inch and preheat the oven to 160℃Fan /170℃/340℉.
Mix the dry ingredients together then add the margarine, eggs, milk and yoghurt and whisk together in a mixer until smooth.
300 g Plain Flour, 60 g Cocoa Powder, 330 g Caster Sugar, 1 tbsp Coffee, 1 tbsp Baking Powder, ½ tsp Salt, 200 g Margarine, 3 Eggs, 180 ml Milk, 2 tbsp Plain Greek Yoghurt
Transfer to the tin and level with the back of a tablespoon. Bake in the oven for 35 minutes. Insert a skewer or knife in the middle and if it comes out clean it's done. Leave to cool in the tin.
Meanwhile, make the frosting. Melt the chocolate in the microwave at 20 second intervals until melted. Leave to cool.
Sieve the icing sugar and cocoa powder in a bowl. Add the margarine and slowly whisk in the milk. Add the melt chocolate and mix until smooth and it forms soft peaks. If it's too thick at a little milk or too runny add some more icing sugar.
260 g Icing Sugar, 6 tbsp Cocoa Powder, 150 g Margarine , 100 ml Milk, 175 g Dark Chocolate
Spread it all on top of the cake with a spatula. Sprinkle with smarties or sprinkles.
Sprinkles/Smarties/Chocolate button for decoration
Slice the cake before serving with a sharp knife, cleaning the knife after each cut.