Preheat the oven to 350 degrees (175 degrees C). Grease a 9x5 inch loaf pan.
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 cornstarch. Pour into the prepared loaf pan.
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 pan.
While the cake is baking, mix together the lemon juice and confectioners' sugar. 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.
