Preheat oven to 350°F (175°C). Line a 9-inch square baking pan with aluminum foil or parchment paper so that the lining overhangs by an inch or two on opposite sides. Lightly grease the lining.
Place the butter and chocolate in a large microwave-safe bowl. Heat in the microwave at half power in 30-second increments until the butter has melted and the chocolate melts when stirred. Stir until smooth. Allow to cool slightly.
Whisk in the cocoa powder, followed by the sugar, and then the eggs and the vanilla. Add the flour, cinnamon, and salt, and whisk until combined.
Stir in the pecans. If you like, reserve a few tablespoons of nuts to scatter over the top of the batter.
Transfer the batter to the prepared pan and spread evenly. Scatter any reserved pecans over the top of the batter. Press down lightly.
Bake 35 to 40 minutes, or until the edges are set and a pick inserted into the center comes out with a few crumbs attached.
Cool the brownies in the pan on a wire rack. Then use the overhanging lining to remove the brownies from the pan. Cut into squares and serve.