Set the oven to 170°C. Line a 23cm cake tin with parchment paper.
Place the sugar, chocolate, butter and water in a saucepan and cook gently until the chocolate and butter have melted. Cool.
Whisk in the buttermilk and eggs. Sift the flour and baking soda together and fold into the mixture.
Spoon into the cake tin and bake for 45 minutes or until an inserted skewer comes out clean. Cool the cake.
Sprinkle the coconut over a tray lined with parchment paper and cook for about 5 minutes in a 200oc oven or until golden brown. Cool.
Whisk the cream to soft peaks, whisk in the icing sugar and 40g of the coconut.
Split the cake in half and spread over half the cream. Top with the remaining cake. Pipe the remaining cream over the top. Sprinkle with the remaining coconut.
Alternatively, rather than cutting the cake in half, spread half the cream on top of the cake and pipe on the remainder.