Freshly grated coconut, coconut flour, coconut palm sugar and coconut milk give a rich, sticky toffee flavour and texture to this cake.
Heat the oven to 180°C/160°C fan/350°F/Gas 4.
Make the sponge. To crack open the coconut, wrap a tea towel around the shell. Set the coconut on a very hard surface, such as a stone floor, and use a hammer to bash and crack the shell. (Try to catch the coconut water and keep it for drinking.) Use a sharp knife to separate the flesh and skin from the shell. Grate the coconut pieces to give 180g. Set aside.
Sift together both flours and the baking powder and salt into a bowl and set aside.
Put the margarine and sugar in the bowl of a stand mixer fitted with the whisk attachment and whisk until light and creamy.
In another bowl, briefly beat together the eggs and vanilla extract to combine. Gradually add this to the creamed mixture, beating well after each addition. Incorporate 1 tablespoon of the flour mixture at each of the last 4 additions, to prevent curdling.
Stir in the grated coconut, then fold in one third of the remaining flour mixture and one third of the coconut milk. Repeat twice more, then transfer the mixture to the prepared tin. Sprinkle the demerara sugar on top.
Bake for about 45–50 minutes, until the cake is well-risen, rich golden brown, and a skewer inserted into the centre comes out clean. Unclip the sides of the tin and leave the sponge to cool on a wire rack.
Meanwhile, make the frosting. Put the butter in a bowl and beat with an electric whisk until light and creamy, then gradually incorporate the icing sugar. Add the milk and vanilla and mix until very light, smooth and spreadable.
Spoon about one fifth of the frosting into a separate bowl and add green colouring until you’re happy with the shade. Carefully spoon and spread the coloured frosting down one side of the piping bag fitted with the star nozzle, then add a similar amount of the plain white frosting to the other side.
Spread the remaining plain frosting over the top of the cake, then pipe green and white rosettes around the edge. Scatter the toasted desiccated coconut over the cake, to decorate.