Preheat oven to 180 C / Gas 4. Grease and flour one 23cm square cake tin. Sieve the flour, cornflour, 200g caster sugar, baking powder, bicarbonate of soda and salt together.
Cream 75g butter. Stir in 100ml buttermilk and 1 teaspoon vanilla. Blend in the flour mixture and beat with an electric mixer on medium speed for 2 minutes. Add chocolate, eggs and the remaining buttermilk. Continue to beat at medium speed for another minute more. Pour batter into the prepared tin.
Bake at 180 C / Gas 4 for 40 minutes. Cool in tin for 15 minutes.
Meanwhile, make the icing. Combine the coconut, dark brown soft sugar, cream, walnuts and 50g butter. Mix until a spreadable consistency is reached. Spread over top of baked cake. Grill 7.5cm from heat until browned (about 3 minutes). Serve cake warm or cold.