Adapted from Martha Stewart Living
8 tablespoons (1 stick) unsalted butter, plus more for pan
6 ounces bittersweet chocolate, chopped
2 cups all-purpose flour
1 teaspoon baking powder
1/8 teaspoon cayenne pepper
1/2 teaspoon salt
1 1/2 cups sugar
4 large eggs
1 tablespoon pure vanilla extract
1 1/4 cups solid-pack pumpkin
1/4 cup vegetable oil
1 teaspoon ground cinnamon
1/4 teaspoon ground nutmeg
1/2 cup chopped hazelnuts or other nuts (optional)
1. Preheat oven to 350 degrees. Butter a 9-inch square baking pan or dish. Cut a length of parchment that will cover the bottom and two sides (makes it much easier to remove), and line the pan with it. Butter the lining as well.
2. Melt chocolate and butter in a heatproof bowl set over a pan of simmering water, stirring occasionally until smooth.
3. Whisk together flour, baking powder, cayenne, and salt in a large bowl; set aside. Put sugar, eggs, and vanilla in the bowl of an electric mixer fitted with the paddle attachment; beat until fluffy and well combined, 3 to 5 minutes. Beat in flour mixture.
4. Pour half of batter (about two cups) into a separate bowl and stir chocolate mixture into it. If you find that it is a little thick (as mine was) add a little more batter (a few tablespoons or so) until it is more pourable. This is important because mine was quite thick, and the pumpkin half was quite thin, so I had trouble swirling the two together.
5. In other bowl, stir in the pumpkin, oil, cinnamon, and nutmeg. Transfer half of chocolate batter to prepared pan smoothing top with a rubber spatula. Top with half of pumpkin batter. Repeat to make one more chocolate layer and one more pumpkin layer. Work quickly so batters don’t set.
6. With a small spatula or a table knife, gently swirl the two batters to create a marbled effect. Be sure to get your knife all the way to the bottom of the pan–I didn’t, and ended up with a chocolate base, not that it is such a bad thing. Sprinkle with nuts, if using.
7. Bake until set, 40 to 45 minutes. Let cool in pan on a wire rack. Cut into 16 squares.