Grease bottom and sides of 8-in. (20-cm) square baking pan or dish with oil or shortening (for best results, use a metal baking pan). Preheat oven to 325°F (160°C) for dark-coloured or non-stick pan, or 350°F (180°C) for glass or metal pan.
Hack Note: Use only the packaged brownie mix, not added ingredients as called for on package.
In medium bowl, whisk dry brownie mix with pumpkin pie spice. Stir in pumpkin purée and egg. Mix until smooth (batter will be thick). Spread batter evenly in prepared pan. Sprinkle on choice of topping, if using.
Bake until toothpick inserted in centre comes out with a few moist crumbs clinging, 34 to 37 min. Remove pan to rack to cool. Just before serving, cut into squares.