pumpkin swirl brownies

Name: pumpkin swirl brownies
Cuisine: American
Ingredients: 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 to 3/4 cups sugar (the original recipe calls for the larger amount; I think it could be dialed down a bit); 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)