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 and sugar until well combined. Add 3 of the eggs and 2 teaspoons of the vanilla, whisking until the mixture is smooth and glossy.
Step 3: Sift in the cocoa powder, flour, and salt. Gently fold the dry ingredients into the wet mixture until just combined. If using chocolate chips, fold them in now.
Step 4: In a separate small bowl, mix the pumpkin puree, the remaining egg, the remaining 1 teaspoon of vanilla, and the pumpkin pie spice until smooth.
Step 5: Spread about 3/4 of the chocolate brownie batter into the prepared pan. Drop spoonfuls of the pumpkin mixture and the remaining brownie batter over the top.
Step 6: Use a knife to swirl the two batters together to create a marble effect. Sprinkle the toasted pecans evenly over the top.
Step 7: Bake for 30-35 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs.
Step 8: Allow the pumpkin pecan brownies to cool completely in the pan before slicing into 16 squares.