Step 1

Arrange a rack in the middle of the oven and heat the oven to 350°F. Coat a 9x5-inch loaf pan with cooking spray.

Step 2

If using walnuts, spread 3/4 cup walnut pieces on a rimmed baking sheet. Bake until fragrant and lightly toasted, about 5 minutes. If the pieces are large, transfer to a cutting board and coarsely chop when cool.

Step 3

Microwave 1 stick unsalted butter in a small microwave-safe bowl in 20-second increments until melted, 40 to 50 seconds total. (Alternatively, melt in a small saucepan over medium heat.) Set aside to cool until just warm.

Step 4

Whisk 1 1/2 cups all-purpose flour, 1 tablespoon pumpkin pie spice, 2 teaspoons baking powder, 3/4 teaspoon kosher salt, and 1/4 teaspoon baking soda together in a bowl.

Step 5

Mash 2 medium bananas in a large bowl with a fork or potato masher until smooth with just a few small lumps (3/4 cup mashed). Add the butter, 1 large egg, 3/4 cup pumpkin purée, 1/2 cup granulated sugar, 1/2 cup packed dark brown sugar, and 1 tablespoon vanilla extract, and whisk until combined.

Step 6

Add dry ingredients to the pumpkin-banana mixture, then stir with a rubber spatula until just incorporated but no dry spots remain. If using walnuts, reserve 2 tablespoons for topping, then fold the remaining into the batter.

Step 7

Pour the batter into the prepared pan and spread into an even layer. Sprinkle with the reserved walnuts.

Step 8

Bake until the bread pulls away from the sides of the pan, the top is cracked, and a toothpick inserted in the center comes out mostly clean with a few moist crumbs attached, 55 to 60 minutes. Let cool 10 minutes in the pan, then flip the bread out onto a wire rack and let cool completely before slicing.