Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper or grease lightly.
In a large bowl, whisk together melted butter and sugar until smooth.
Add eggs and vanilla extract to the butter mixture and mix well.
Divide the mixture in half into two separate bowls.
In the first bowl, add cocoa powder, flour, salt, and baking powder. Stir until combined to form the brownie batter.
In the second bowl, combine pumpkin puree, egg, sugar, cinnamon, and nutmeg. Mix until smooth to form the pumpkin swirl batter.
Spread half of the brownie batter into the prepared pan.
Dollop the pumpkin batter evenly over the chocolate layer.
Spoon the remaining brownie batter on top and gently swirl with a knife or skewer. Do not overmix.
Bake for 30–35 minutes or until a toothpick inserted in the center comes out with a few moist crumbs.
Let cool in the pan for at least 15 minutes before slicing and serving.