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1
Preheat the oven, with a rack in the middle position, to 350u00b0F. Butter and flour a 9x5-inch loaf pan and set aside. In a large bowl, with an electric beater, cream together the butter and the granulated and brown sugars until fluffy and light. Beat in the eggs, one at a time, until incorporated. Add the mashed banana, banana liqueur (or vanilla extract), and sour cream. Beat at low speed until combined. Drizzle in the walnut oil and beat until the mixture is smooth and the oil is absorbed.
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2
In a medium bowl, whisk together the flours, baking powder, baking soda, and salt. Add the dry ingredients to the banana mixture in two additions, beating at the lowest speed. Before the flour is entirely incorporated, switch to a large spatula and gently stir in the walnuts. Scrape the bottom and sides of the bowl to mix in all the dry ingredients, then transfer the batter to the prepared loaf pan. Smooth the surface of the batter and place the pan in the oven.
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3
Bake the banana loaf for about 60 minutes, or until handsomely golden brown and firm to the touch. A tester inserted into the middle of the loaf should come out clean. If you find the bread browning too quickly, drape a sheet of foil over the top of the loaf. You don't want the edges to burn.
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4
When done, remove to a rack to cool, in the pan, for 15 minutes. Then turn the loaf out onto the rack and cool completely before cutting and serving. Store, tightly wrapped, at room temperature for up to 4 or 5 days. In fact, if you can wait, let the loaf rest overnight before serving.
