Sieve flour, salt and baking powder into a bowl. Rub in the butter until mixture resembles breadcrumbs. Add sugar, coconut and zest of a lime.
In a separate bowl, combine egg and add 100ml of the milk and vanilla.
Make a well in the middle of the flour mixture. Add the liquid ingredients and mix well to form a stiff consistency, pouring in the extra milk if necessary.
Transfer to a greased 15cm (6 inch) cake tin and bake at 170° C / 325°F /Gas 3 for 25-30 minutes. Leave to cool slightly in the tins before removing to a wire rack to cool.
Put the egg whites, sugar and 75ml of water in a heat proof bowl over a saucepan of simmering water and beat gently with an electric handheld whisk until stiff peaks form. This will take about 6-7 minutes. Remove from the heat and mix with vanilla extract. The finished result should look thick and shiny.
Using a sharp knife cut the cake into two or three layers and sandwich together with the frosting. Cover the remaining frosting over the top and sides of the cake and sprinkle with a little coconut. Top with a glace cherry.