Position oven rack in the lower third or the oven; preheat oven to 350°.
Butter and flour a 9-inch square baking pan; set aside.
In a bowl, whisk the cocoa powder, flour , cinnamon, baking powder, nutmeg, and salt until well combined; set aside.
In a big bowl, cream the brown sugar and butter using an electric mixer on medium speed; continue beating until the mixture is a pale brown and thick, about 5 minutes.
beat in the eggs one at a time, adding the second only after the first is thoroughly incorporated.
Add in the canned pumpkin and continue beating at medium speed until smooth and creamy, about 3 minutes.
With a wooden spoon or rubber spatula, stir in the flour just until incorporated; do not beat.
Pour the batter into prepared pan, spreading it gently to the corners.
Bake for 30 minutes, or until a pick comes out clean; set pan on a wire rack to cool for at least 30 minutes.
Cut brownies into 16 squares while they are still in the pan.
Carefully remove them with an offset spatula.
Serve immediately or let cool completely before covering with plastic wrap for storage at room temperature.
*May add one of the following with the pumpkin: 2 teaspoons vanilla extract, 1 teaspoon maple extract, 1 teaspoon rum extract and/or ¾ cup of any of the following: chopped pecans, chopped walnuts, semisweet chocolate chips, white chocolate chips.