Tablespoons (1 stick) unsalted butter, plus more for pan
Ounces bittersweet chocolate, chopped
Cups all-purpose flour
Teaspoon baking powder
/4 teaspoon cayenne pepper
/2 teaspoon salt
/4 cups sugar
Tablespoon pure vanilla extract
/4 cups solid-pack pumpkin
/4 cup vegetable oil
Teaspoon ground cinnamon
/4 teaspoon ground nutmeg
/2 cup chopped hazelnuts or other nuts
Preheat oven to 350 degrees. butter a 9-inch square baking pan or dish. line bottom of pan with parchment paper; butter lining.
Melt chocolate + butter in a heatproof bowl set over a pan of simmering water, stirring occasionally until smooth.
Whisk together flour, baking powder, cayenne, + salt in a large bowl; set aside. put sugar, eggs, + vanilla in the bowl of an electric mixer fitted with the paddle attachment; beat until fluffy + well combined, 3 to 5 minutes. beat in flour mixture.
Divide batter between two medium bowls (about 2 cups per bowl). stir chocolate mixture into one bowl. in other bowl, stir in pumpkin, oil, cinnamon, + 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 + one more pumpkin layer. work quickly so batters don’t set.
With a small spatula or a table knife, gently swirl the two batters to create a marbled effect. sprinkle with nuts if using them.
Bake until set, 40-45 minutes. let cool in pan on a wire rack. cut into 16 squares.