Preheat oven to 180 °C (350°F). Line 20x20 cm (8x8 inch) baking pan with baking paper. Set aside.
Make the pumpkin swirl mixture: Add all the ingredients in the bowl and with an electric mixer, beat until smooth. Set aside.
Make the brownie mixure: In a bowl whisk together flour, cocoa powder and salt. Set aside. Add chocolate, butter and sugars in a heatproof bowl and set over a pan of simmering water, stirring occasionally until melted and smooth, and mixture is glossy, about 4-5 minutes. We want sugars to melt a bit too.
Allow mixture to cool slightly, then add in vanilla and eggs one at a time, whisking in between each addition.
Fold in dry ingredients until just combined.
Pour 3/4 of the brownie batter into the bottom of prepared pan and smooth into an even layer. Drop spoonfuls of pumpkin swirl batter onto brownie batter in pan. Then add spoonfuls of the rest of brownie butter over pumpkin mixutre. With a small spatula or a table knife, gently swirl the two batters to create a marbled effect.
Bake 30-40 minutes until a toothpick comes out mostly clean (there should be some moist crumbs since we want brownies to be fudgy). Allow to cool completely before cutting into bars. Store in airtight container.