• Butter for the pan
• 2 cups walnut pieces

Dry Mix

• ½ cup quinoa flour
• 1 cup all-purpose flour
• 1 tablespoon baking powder
• 1 teaspoon kosher salt

Wet Mix

• 4 ounces (1 stick) cold unsalted butter, cut into ½-inch pieces
• ½ cup dark brown sugar
• ½ cup sugar
• 3 ripe bananas, about 1¼ pounds
• 2 eggs
• ⅓ cup sour cream
• 1 teaspoon pure vanilla extract

1. Position a rack in the middle of the oven and preheat to 350°F. Rub a 9-inch round cake pan with butter.
2. Measure the walnuts onto a baking sheet and toast in the oven until light goldenbrown and fragrant, about 15 minutes. Set aside to cool.
3. Sift the dry ingredients into a large bowl, pouring back into the bowl any bits of grain or other ingredients that may remain in the sifter, and set aside. Grind 1 cup of the toasted, cooled walnuts in a food processor, about 15 seconds. With a sharp knife, finely chop the remaining cup of walnuts and reserve. The chopped nuts will be added later as a topping.
4. Add the butter and sugars to the bowl of a standing mixer. Attach the bowl and the paddle and mix on medium-high speed until light and creamy, about 3 minutes. Using a spatula, scrape down the sides and the bottom of the bowl. Add the bananas and mix on medium speed for about 30 seconds, or until the bananas are broken down. Add the eggs, one at a time, and mix on medium speed, until thoroughly combined. Add the sour cream and vanilla and mix until combined. Again, scrape down the sides and the bottom of the bowl.
5. Add the ground walnuts and the dry ingredients to the mixing bowl and mix on low speed until just combined. Remove the bowl from the mixer, scrape down the sides, and stir the batter until uniformly combined.
6. Using a spatula, scrape the batter into the buttered pan and smooth the top. Sprinkle the chopped walnuts evenly over the top, out to the very edges.
7. Bake for 50 to 60 minutes, rotating the pan halfway through, until the cake is golden brown and springs back when lightly touched, or a skewer inserted in the center comes out clean. Allow the cake to cool in the pan. It should be eaten at room temperature. Wrapped tightly in plastic, the cake can be kept for up to 3 days.