Preheat the oven to 180°C, fan 160°C, gas 4. Grease and line a 24cm round cake tin.
First make the sponge cake. In a mixing bowl, beat the egg whites with a pinch of salt until stiff peaks form. Sift the flour and baking powder into another mixing bowl. When the egg whites are stiff, add the sugar and carry on whisking. Once the mixture is firm, add the egg yolks, whisking continuously. Add the flour/baking powder and the spices and carry on whisking for another 2-3 mins. Pour the batter into the prepared tin and place in the oven for 20-25 mins, until well risen with a nice golden top. Remove the cake from the oven and leave to cool in its tin.
To make the coconut cream, pour the coconut milk, condensed milk, cornflour and water in a saucepan and simmer for a few minutes, stirring constantly, until thick and creamy. Add the rum and lime zest and leave to cool.
When the cake is cool, turn it out on to a piece of greaseproof (or waxed) paper. Slice it in half horizontally, with the cut sides facing up. Sprinkle some rum over the cut sides. Spread some of the coconut cream evenly over both sides, then sandwich them together. Spread the remaining coconut cream over the top of the cake and all around the sides.
To decorate, break open the coconut and scoop out the flesh. Peel and discard the brown skin. Wash the coconut flesh under cold water before grating it finely. Sprinkle all over the cake until completely covered and scatter glacé cherries on top, if desired. Place it in the refrigerator for at least 4 hours before serving.