Pumpkin-Swirl Brownies

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
• 1/2 teaspoon salt
• 1 3/4 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 (I used pecans)