Heat oven to 180C/fan 160C/gas 4. Butter and line the base of a 20cm square baking tin. Beat together the butter and sugar until pale and creamy, then beat in the eggs. Add 1 tbsp of the flour if the mix starts to curdle.
Sieve in the flour, baking powder and cocoa and fold in with a metal spoon. Stir in the milk. Scrape mix into the tin and level the top. Bake for 18-20 mins or until the cake springs back when pressed. Allow to cool in the tin.
To make the icing, put the chocolate, butter and 4 tbsp water in a pan and gently heat until melted. Allow to cool slightly, then beat in the icing sugar.
Remove cake from tin and peel away paper. Cut into 16 squares. Dip the squares into the icing, then roll in the coconut. Allow to set on cooling rack.