Preheat the oven at 210°C (410°F) or 190°C (475°F) for fan-forced ovens. Line and grease a square 20cm (8 in) tin with nonstick baking paper.
Using a stand mixer or a handheld mixer cream the sugar and the egg on high speed until pale for approximately 5 minutes. Add the egg in one at a time and beat until combined. Adjust the mixer speed to low, add in the vanilla extract, baking powder, half of the flour and half of the milk. Mix for 30 seconds then add in the remaining flour and milk, mix until all ingredients are combined.
Transfer the batter into the prepared tin, using a butter knife spread the batter evenly. Bake for 15 minutes or until a skewer comes out clean when inserted in the middle. Leave the cake to cool in the tin for 10 minutes then carefully transfer it to a large plate or chopping board. Freeze the cake uncovered for 30 minutes.
Carefully cut the edges off the cake, removing as little as possible but enough to make a clean cut. Cut the cake in half making two even rectangles. Thinly slice the top of the cakes. Put the cake on top of each other and trim any bits needed to make it even. Freeze the cakes uncovered for 15 minutes.
Place the desiccated coconut on to a large flat plate and spread it out evenly. In a bowl combine the icing sugar, cocoa, butter and the boiling water, stir thoroughly to combine.
Pour in the chocolate mixture into a rectangular container or baking dish that is slightly bigger than the cake. Dip one cake at a time into the icing mixture ensuring the whole cake is covered. Quickly transfer the cake on top of the coconut, use a spoon to help toss the coconut all over the cake. Place the cakes on a cooling rack and freeze for 15 minutes to set the icing.
On one side of a cake spread the on the jam evenly. Whip the cream until piping consistency, spread or pipe the cream over the jam. Carefully place the cake on top of the filling. If the cake has not fully defrosted leave it at room temperature for 10-15mins prior to serving.