Preheat oven to 350. Line or grease and flour 1 12 cupcake cupcake pan. In a small saucepan, heat butter, coca cola and rum to boiling. Remove from heat and allow to cool. Mix together lime juice and milk in a small bowl* or measuring cup and set aside. Meanwhile, whisk together cocoa, flour, sugar, baking soda, baking powder and salt. Add the cooled butter/coke mixture and beat until well combined. Add egg, beat then pour in the milk. Mix until combined. The batter will be rather thin. Fill each well 2/3 of the way full. Bake 20 minutes or until a toothpick comes out clean. Remove cupcakes to wire racks as soon as possible to cool. Frost with Chocolate Rum Cream Cheese Frosting when cool. Garnish with a wedge of lime.