1. Preheat oven to 350°F. Line an 8x8 inch baking pan with parchment paper and grease it.
2. In a large bowl, whisk melted butter and 1 cup sugar. Whisk in eggs one at a time, then vanilla bean paste.
3. In a separate bowl, whisk flour, cocoa powder, baking powder, and salt. Add dry ingredients to wet ingredients and mix until just combined. Stir in chocolate chips if using.
4. In a medium bowl, combine pumpkin puree, 2 tablespoons sugar, egg yolk, pumpkin pie spice, vanilla bean paste, and 1 tablespoon flour. Mix until smooth.
5. Pour two-thirds of the brownie batter into the prepared pan. Drop spoonfuls of the pumpkin mixture over the batter. Drop the remaining brownie batter over the pumpkin mixture.
6. Use a knife to gently swirl the batters together.
7. Bake for 30-35 minutes, until edges are set and a toothpick inserted into the brownie comes out with moist crumbs.
8. Cool completely in the pan on a wire rack before lifting out. Cut into squares.