Make the cashew dough ahead of time by creaming the butter with the sugar. Sift the dry ingredients together, then add after you have added the eggs to the butter mixture. Add the rum last. Wrap the dough and refrigerate. Prepare a half sheet pan and line with paper. Roll out half of the dough on the lined sheet pan about 1/8 inch thick; refrigerate. Spread the guava jelly over the dough. Soften the rest of the dough and place in a pastry bag with a small round tip, about 1/4 inch in diameter. Pipe a rough pattern over the guava jelly. Make sure that some of the dough touches each other. Bake in a 350o oven for 15 to 25 minutes until golden brown and make sure that the bottom is completely cooked. Once cool, cut the tart in squares. Dust lightly with powder sugar.