Serving espresso with a lemon slice is traditional in Italy, so it's not too much of a stretch to imagine how this drink, which builds on that idea, will be utterly delicious. While you brew the espresso, in a pot, combine brown sugar (light or dark, according to your taste) with a lemon that's been cut into pieces. Then pour in the espresso, water, and rum (if desired), and serve over crushed ice.