Preheat oven to 350°F. Line two muffin tins with 18 cupcake liners.
Stir flour, baking powder, baking soda, and salt in medium bowl. Set aside.
Cream butter and sugar on medium speed for 3 minutes or until light and fluffy. Turn off mixer and scrape down sides.
Add eggs, one at a time. Let stand mixer run for 30 seconds after the addition of each. Scrape down sides after adding both eggs.
Mix lime juice, grapefruit juice, grenadine, cinnamon, fennel seeds, falernum, rum, and bitters together in small bowl. Set aside.
Whisk milk, sour cream, and vanilla extract in small bowl. Set aside.
Turn mixer to low speed. Add 1/2 of flour mixture to bowl. When combined, add lime mixture and milk mixture bowl. When combined, add remaining flour mixture.
Turn off mixer and scrape down sides.
Add green food coloring if desired.
Bake for 15-18 minutes or until cupcakes bounce back when pressed. Let cool completely on cooling rack before frosting.