Get the pan ready: Heat your oven to 350°F. Line an 8×8-inch pan with parchment paper, leaving a little overhang for easy lifting later. Lightly grease the surface.
Melt the chocolate and butter: Place the butter and chopped chocolate in a microwave-safe bowl. Heat in the microwave for 30 seconds, then stir well. Continue heating in 10-second intervals, stirring after each, until smooth and fully melted.
Whisk the wet ingredients: In a mixing bowl, beat the sugar and eggs with a hand mixer until the mixture is pale, light, and fully combined.
Add the dry ingredients: Gently mix in the flour, baking powder, and salt, stirring just until no dry streaks remain.
Make the chocolate batter: Transfer half of the batter into a separate bowl. To the first bowl, stir in the melted chocolate mixture until fully combined.
Make the pumpkin batter: In the second bowl, gently fold in the pumpkin purée, oil, cinnamon, and pumpkin pie spice until evenly mixed.
Swirl it together: Spread the chocolate batter evenly into the prepared pan to form the bottom layer. Carefully spoon the pumpkin batter on top, smoothing it into an even layer. Using a skewer or toothpick, gently drag through both layers to create a marbled swirl pattern.
Bake: Slide the pan into the oven and bake for 30–35 minutes, or until a toothpick inserted near the center comes out with just a few moist crumbs clinging to it.
Cool & cut: Place the pan on a wire rack and let the brownies cool completely before lifting them out and slicing into squares. The flavors deepen as they rest, making each bite even more delicious. Serve and enjoy!