When the cake is almost done, prepare the glaze by boiling and stirring the sugar, butter, lemon juice and lemon rind for 2-3 minutes. Remove from heat, let cool slightly, and then stir in the rum. Remove cake from oven and spoon hot glaze over the bottom, but mostly between the cake and the sides (and center) of the pan. Let soak in and carefully turn cake out of pan when cool.