Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking pan or line it with parchment paper for easy removal.

In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt. Set aside.

In a large bowl, combine the melted butter and granulated sugar. Stir until well combined. Add the eggs and vanilla extract, mixing until smooth.

Gradually add the dry ingredients to the wet mixture, stirring until just combined. Do not overmix.

In another bowl, mix the pumpkin puree, pumpkin pie spice, and brown sugar until smooth.

Pour half of the brownie batter into the prepared baking pan. Spread it evenly. Spoon dollops of the pumpkin mixture over the brownie batter, then pour the remaining brownie batter on top. Use a knife or toothpick to gently swirl the pumpkin mixture into the brownie batter.

Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Allow the brownies to cool in the pan for at least 15 minutes before cutting them into squares.