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.