Pour water to a depth of 1" in a 4-qt. saucepan over medium heat and cover with a heatproof bowl. (Or, if you have one, set up a double boiler over medium heat.) Add chocolate and warm until melted. Add butter and, using a wooden spoon, stir until combined. Stir in cream until glossy and add Kahlua, cinnamon, salt, and cayenne. Serve fondue with cut fruit, marshmallows, or brownies, or drizzle over ice cream.