Combine the flour, baking powder, salt, sugar, coconut, lemon rind and melted butter in a bowl and mix well. Stir in the milk, buttermilk, and vanilla extract. Be gentle when mixing and do not overbeat. Overbeating will result in a dry texture of the baked cake.
Spread the mixture evenly into a greased and lined 12 x 9 inch ( 30 x 21 cm ) pan and bake in a preheated oven of 340°F / 170°C for 20 to 25 minutes until baked or a skewer inserted comes out clean.
Leave the cake in the tin to cool completely before frosting/icing.
TO MAKE THE FROSTING / ICING
Place the powdered sugar/icing sugar and butter in a bowl and pour in the boiling water to make a thick frosting/icing. If you find that the frosting is too thick, then just add a little more water. A teaspoon at a time, until you achieve the desired consistency. Keep in mind that the frosting will set firm once it has cooled down. If the frosting is too runny, then add in a bit more powdered/icing sugar. Spread the warm frosting over the cake.
Sprinkle with lightly toasted desiccated coconut if preferred.
Notes
If you do not have buttermilk, then substitute it by adding 4 teaspoons of lemon juice or vinegar to 1 cup (250ml) full fat / full cream milk. Let it stand for 5 minutes before using it in place of the buttermilk.
Make this frosting only once the cake has cooled. The warm frosting has to be spread immediately on the cooled cake. If you allow the frosting to cool, it becomes clumpy and difficult to use.