Preheat the oven to 180°C, fan 160°C, gas mark 4 and grease and line a 31x20cm shallow tin.
In a bowl, beat together the butter and sugar until light and fluffy. Add the eggs one at a time, beating well after each addition. Add the sifted flour, baking powder and cocoa and fold in.
Turn into the prepared tin, push into the corners and level out the top. Bake in oven for 25-30 minutes (test with a skewer, it should come out clean if cake is cooked). Cool and place on a wire rack.
Sift the icing sugar and cocoa into a bowl and add a little water (a few teaspoons) to make a spreadable icing. Spread over the top of the cooled cake and top with smashed pieces of creme egg.
All information included in this recipe has been provided by Mondelez - Cadbury.