Step 1: Preheat your oven to 350°F (175°C) and grease an 8x8 inch baking pan or line it with parchment paper.
Step 2: In a large bowl, whisk together the melted butter and brown sugar until smooth. Add the egg and vanilla, whisking until well combined.
Step 3: In a separate bowl, whisk the flour, baking powder, and salt. Gradually stir the dry ingredients into the wet ingredients until just combined.
Step 4: Divide the batter into two equal portions in separate bowls.
Step 5: Into the first bowl, stir in the pumpkin puree and pumpkin pie spice. Into the second bowl, stir in the cocoa powder and melted chocolate.
Step 6: Drop alternating spoonfuls of the pumpkin and chocolate batters into the prepared pan. Use a knife to swirl the batters together gently.
Step 7: Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs. Let cool completely before slicing.