Step 1: Preheat your oven to 350°F (175°C) and grease an 8×8 inch baking pan or line it with parchment paper 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 egg and vanilla extract, whisking vigorously for about 1-2 minutes until the mixture lightens in color.
Step 4: Stir in the pumpkin puree until fully incorporated.
Step 5: Gently fold in the flour, pumpkin pie spice, baking powder, and salt using a spatula until just combined. Do not overmix.
Step 6: Transfer 1 cup of the batter to a separate small bowl. Stir the cocoa powder and melted chocolate chips into this smaller portion until smooth.
Step 7: Drop spoonfuls of the plain pumpkin batter and the chocolate batter into the prepared baking pan, alternating colors.
Step 8: Use a knife or skewer to gently swirl the two batters together to create a marbled pattern.
Step 9: Bake for 28-32 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs.
Step 10: Allow the pumpkin and chocolate swirl blondies to cool completely in the pan before slicing into 16 squares.