Preheat the oven to 180°C, fan 160°C, gas 4. Grease and line a deep 23cm cake tin.
Put the butter and sugar in a large mixing bowl and, using an electric hand whisk, beat until light and fluffy.
Keeping the whisk running, add in the beaten eggs, a little at a time, with a spoonful of the flour, making sure the egg is fully combined after each addition.
When all the egg is incorporated, mix in the vanilla extract and sift over the remaining flour and the cocoa powder. Carefully fold the flour and cocoa into the cake mixture until you have a smooth mixture with no lumps.
Spoon the mixture into the lined cake tin and bake on the middle shelf of the oven for 1 hour or until the cake is risen and springs back to the touch. Turn the cake out onto a wire rack and allow to cool completely. Cut in half horizontally.
Meanwhile, make the buttercream icing. In a large mixing bowl, beat the butter using an electric hand whisk until smooth and softened. Sift over the icing sugar and beat it into the butter on a low speed until combined. Increase the speed and continue to beat until you have a smooth butter icing.
Sandwich the two cake halves together with one-third of the buttercream. Spread the remaining buttercream over the top and sides of the cake.
Dust a worktop with a little icing sugar. Roll out the chocolate ready to roll icing to make a circle about 5mm thick – it should be big enough to completely cover the cake. Drape it over the rolling pin and transfer to the cake. Carefully smooth over the top and sides of the cake. Trim the icing and use the trimmings to cut out a smiley mouth, 2 eyebrows and two nostrils. Make two eyes by rolling a small amount of the chocolate icing into little balls. Use the remaining trimmings to make the chimp’s ears.
Now roll out the ivory icing to a thickness of 5mm then use to create the chimp’s face. Begin by cutting an oval shape for the bottom of the face, then cut out a shape like a pair of upside-down sunglasses for the top half of the face. Position them on the cake. Cut out 2 half moon shapes to go on top of the ears. Put the eyebrows, eyes, nostrils and mouth into position to finish the cake.
Cook's tip: store the iced and decorated cake in an airtight container for up to 5 days. The undecorated cake can be frozen for up to 3 months. Simply wrap in cling film or put into an airtight container before putting in the freezer.