Preheat oven to 350 F and line muffin tins with cupcake liners (makes 24 cupcakes).
Coarsely chop the chocolate and place in a glass or metal bowl over a simmering pot of water. Mexican chocolate contains sugar and cinnamon and will not melt smooth and shiny the way regular chocolate does. Just keep checking it and gently mashing it with a heavy rubber spatula. It will melt into more of a paste-like consistency. Turn off heat and place chocolate in a room temperature bowl to cool while to make the batter.
In mixer bowl combine cake mix, spices, buttermilk, butter, eggs and sour cream. Beat on lowest speed for 30 seconds to moisten. Turn off mixer and add the chocolate. Beat again on low speed for 15-20 second to mix in the chocolate. Turn off mixer and scrape down sides. Turn mixer on medium speed and beat for two minutes.
Fill muffin cups about 3/4 full and bake for 22-26 minutes. Allow to cool for 10 minutes in pan. Remove from pan to wire rack. Cool completely before frosting.