Cake: Preheat the oven to 180 degrees C. Grease a 23x13cm loaf tin.
In a large bowl, beat the butter and sugar together until light and fluffy. Beat in the eggs one at a time, mixing well after each one. Stir in the rum, then mix in the flour and cornflour. Pour into the prepared loaf tin.
Bake for 1 hour and 15 minutes in the preheated oven or until a knife inserted into the crown comes out clean. Cool for at least 10 minutes before removing from the tin.
Lemon Glaze: Mix together the lemon juice and icing sugar.
To Serve: When the cake comes out of the oven, poke with a long fork or knitting needle all over. Pour the glaze over the top and let it soak in. Cut into slices to serve.