Step 1: Preheat your oven to 350°F (175°C) and line an 8x8 inch baking pan with parchment paper, leaving an overhang for easy removal.
Step 2: In a large bowl, whisk together the melted butter, brown sugar, and granulated sugar until well combined and smooth.
Step 3: Add the eggs one at a time, whisking well after each addition, then stir in the vanilla extract.
Step 4: Gently fold in the flour, baking powder, and salt until just combined. Do not overmix.
Step 5: Divide the batter into two equal portions in separate bowls.
Step 6: In the first bowl, stir in the pumpkin puree and the pumpkin pie spice until the color is uniform.
Step 7: In the second bowl, stir in the cocoa powder and the melted chocolate chips until rich and dark.
Step 8: Drop alternating spoonfuls of the pumpkin and chocolate batters into the prepared baking pan.
Step 9: Use a butter knife or skewer to swirl the two batters together in a figure-eight pattern to create a marbled look.
Step 10: Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs.
Step 11: Allow the blondies to cool completely in the pan before lifting them out and slicing into 16 squares.