Heat oven to 160°C/320°F/gas 4. Grease and line the base of a 20cm square baking tin.
Beat the butter and sugar together until pale and creamy, then beat in the eggs.
Sieve in the flour, baking powder and cocoa and fold into the mixture using a large metal spoon. Next, gently stir in the milk.
Transfer the mixture into the tin and level using palette knife.
Place the cake in the oven to bake for 18-20 mins or until the cake is firm.
Remove from the oven and leave to cool in the tin.
To make the icing, add the chocolate, butter and 4 tbsp water into a saucepan and gently heat until melted.
Allow to cool slightly and then beat in the icing sugar.
Remove cake from tin and remove the greaseproof paper.
Cut into squares and dip each square in the icing and then roll in coconut. Leave to cool on a cooling rack.