Preheat the oven to 350°F. Grease a 9x13-inch square baking pan, or spray generously with nonstick baking spray.
In a medium saucepan, heat the butter and unsweetened chocolate over low heat, stirring frequently, until the chocolate and butter have melted. Remove from the heat and stir in the sugar, then the cocoa powder. Beat in the eggs one at a time.
Stir in the vanilla extract, then the flour until completely incorporated. Stir in the cinnamon and cayenne until blended. Scrape the mixture into the prepared pan and bake for 25 to 30 minutes, until a wooden skewer inserted into the middle of the pan comes out clean.
Cool completely (really, completely) on a wire rack. Cut into 24 squares.
Notes
If you’re wondering what makes this brownie recipe Mexican Hot Chocolate, it’s the addition of cinnamon and a bit of cayenne, both adding warmth and a touch of actual heat. The size of the pinch of cayenne should be substantial, but the first time around exercise some restraint; you will be making these again, and you will have the chance to add more the next go round.