Melt the butter and chocolate separately: Cut the butter into small pieces and melt it in the microwave, then set aside; in a separate bowl, melt the dark chocolate (using a microwave or double boiler) until smooth and let it cool slightly.

Whisk the eggs and sugar: In a large bowl, whisk the eggs and sugar together until the mixture is light, fluffy, and pale, about 3-5 minutes.

Combine wet and dry ingredients: Add the melted butter to the whisked egg mixture, mix well, then gently fold in the flour, baking powder, and salt until smooth and lump-free.

Divide the batter: Pour half the batter into a separate bowl; to one portion, mix in melted chocolate and cocoa powder until smooth, and to the other, stir in pumpkin puree, vanilla, and pumpkin spice mix until fully combined.

Assemble the brownie layers: Line a baking pan with parchment paper, pour in the chocolate batter as the base, then gently spread the pumpkin batter on top; for a marbled effect, swirl the two layers with a toothpick or knife.

Bake: Preheat the oven to 350°F (180°C) and bake the brownie for 25-30 minutes, checking with a toothpick — it should come out with a few moist crumbs.

Cool and serve: Allow the brownie to cool completely in the pan; for cleaner slices, refrigerate for 1-2 hours before cutting.