Preheat the oven to 350°F (180°C) and butter and line with parchment a 9x13-inch (23x33- cm) baking pan.
To make the chocolate layer: In a medium bowl, whisk together the sugar, butter, eggs, and vanilla until well blended.
In a separate medium bowl, whisk together the flour, cocoa powder, salt, and baking powder.
Mix the wet and dry ingredients together, then fold in the chopped chocolate and spread evenly in the prepared pan.
To make the pumpkin layer: In a medium bowl, whisk together the sugar, pumpkin puree, butter, eggs, and vanilla extract.
In a separate bowl, whisk together the flour, pie spice, baking powder, and salt, then mix the wet into the dry ingredients until just combined.
Pour the pumpkin batter on top of the brownie batter and gently swirl the batters together with a skewer for just a few times until they are marbled.
Bake for 25-30 minutes, until set and a wooden skewer inserted in the center comes out mostly clean but with a few moist crumbs still attached.
Let cool completely before cutting into 24 squares.
Store leftovers in an airtight container at room temperature for up to 3 days or freeze for up to 2 months. Defrost at room temperature for about an hour.