Preheat your oven to 350°F (175°C) and lightly grease an 8x8-inch baking pan or line it with parchment paper.

In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt until combined.

In a large bowl, combine melted butter and sugar, whisk until creamy, then add eggs one at a time, followed by vanilla and pumpkin puree.

Gradually add the dry ingredients to the wet mixture, folding gently until just combined.

Pour half of the brownie batter into the prepared pan, dollop the remaining batter on top, and swirl with a knife.

Bake for 25-30 minutes, checking for doneness with a toothpick; it should come out clean or with a few moist crumbs.

Cool in the pan for 10-15 minutes, then cut into squares and serve.