Position a rack in the middle of the oven and preheat to 350°F (180°C). Grease an 8×8-inch (or 9×9-inch) metal pan then line with parchment paper, leaving an overhang on all sides.
Combine the chopped chocolate, cocoa powder, and espresso powder in a heatproof liquid measuring cup or medium bowl and set aside.
170 g dark chocolate, 20 g unsweetened natural or Dutch process cocoa powder, ½ tsp espresso powder
In a medium bowl, whisk together the flour, baking powder, and salt, then set aside.
240 g all-purpose flour, 1 tsp baking powder, ½ tsp fine sea salt
In the bowl of a stand mixer fitted with the whisk attachment, using a handheld mixer, or using a balloon whisk by hand, whisk together the sugars, eggs, and vanilla until pale, fluffy, and well combined, about 3 to 5 minutes. Stir in the flour mixture until just combined.
200 g granulated sugar, 110 g light brown sugar, 4 large eggs, 1 tbsp vanilla extract
Divide the batter in half between two medium bowls. Stir in the pumpkin puree, vegetable oil, and pumpkin pie spice into one of the bowls until well combined, then set aside.
285 g pumpkin puree, 60 g vegetable oil, 1½ tsp pumpkin pie spice
Add the butter to a small saucepan over medium heat and cook until the butter just comes to a vigorous simmer, about 2 to 3 minutes, swirling the pan occasionally. Immediately pour the hot butter over the chocolate mixture and let sit for 2 minutes. Whisk until the chocolate is completely smooth and melted, then stir the melted chocolate mixture into the other bowl.
115 g unsalted butter
Transfer half of the chocolate batter to the prepared pan. Top with two-thirds of the pumpkin batter. Repeat to make one more chocolate layer and one more pumpkin layer.
With a small spatula or butter knife, gently swirl the two batters to create a marbled effect. Bake until set, 30 to 35 minutes for a 9x9-inch pan; 40 to 45 minutes for an 8x8-inch pan. Let cool in pan on a wire rack, then cut into squares and ENJOY!
*If you don't have pumpkin pie spice on hand, replace with: 1 teaspoon ground cinnamon, ¼ teaspoon ground nutmeg, ⅛ teaspoon ground ginger, and ⅛ teaspoon ground cloves.