Special equipment: A small paint brush, small paper mini cupcake or candy cups, a candy thermometer, optional
Coat a large shallow pan with some of the butter. Bring the condensed milk, 1 tablespoon butter, the cocoa powder, cinnamon and cayenne, if using, to a boil in a large saucepan over medium heat, stirring constantly with a rubber spatula. Reduce the heat to medium-low and cook, stirring constantly, until the mixture pulls away from the sides of the pan and has the consistency of very thick frosting, 15 to 20 minutes. Pour the mixture into the greased pan; let cool completely and become firm, about 1 hour. (You can refrigerate it to speed up the process).
Line a baking sheet with wax paper. Grease your hands with butter and shape the cooled truffle mixture into 1- to 2-inch balls and place on the wax paper.
For the decorations: You can use melted or tempered chocolate to coat the truffles before decorating. Tempering is the key to shiny, smooth-looking chocolate that remains solid at room temperature.
To melt the chocolate without tempering, put the pieces in the top of a double boiler or in a heatproof bowl set over, not in, a pot of simmering water and stir until melted.
To temper the chocolate, melt about 2/3 of the chopped chocolate in the top of a double boiler or heatproof bowl that fits snugly over, not in, a saucepan of water. Make sure the bottom of the bowl does not touch the water. Bring the water to a simmer over medium-low heat, stirring constantly as the chocolate melts. Once the chocolate reaches 115 degrees F. on a candy thermometer, remove the bowl of melted chocolate from the double boiler and stir in the remaining chocolate to reduce the temperature. Keep stirring until the temperature reaches 85 degrees F., then pick out any chunks of unmelted chocolate. Put the chocolate back over the simmering water and stir until the temperature reaches 90 degrees. Keep the chocolate at 90 degrees F. while you decorate the truffles.
Decorate each truffle ball as described below and return to the wax paper. Refrigerate for at least 1 hour before serving. Transfer to mini cupcake liners or paper candy cups and serve.
Lump sugar, also called rock sugar, is available in most Asian markets. Quins are colorful edible shapes such as flowers and stars cut from sheets of tinted sugar. They are larger, flatter and crunchier than standard round sprinkles and are available at many cake decorating and craft stores, and on-line at Amazon and Bake it Pretty.