Melt the butter in a saucepan.
Stir in water and sugar. Boil for 5 minutes, stirring constantly.
Remove mixture from the heat and stir in the rum to make the glaze.
Immediately Drizzle and smooth glaze evenly over the top and sides of cake while the cake is still warm. (save some glaze for the top)
Allow to sit for five minutes.
Invert the cake onto a serving plate.
Slowly drizzle remaining glaze all over the top of the cake, allowing it to drip down the sides.
Cool to room temperature before serving to ensure glaze has soaked in completely.