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1
Preheat the oven to 350 degrees. Butter a 10-inch tube pan; dust with flour, tapping out the excess, and set aside.
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2
To make the walnut and sugar sprinkle, combine the walnuts, sugar, and cinnamon in a small bowl and set aside.
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3
To make the banana batter, sift together the flour, baking powder, and salt onto a sheet of waxed paper and set aside.
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4
In a large bowl, cream the butter with an electric mixer on high speed until it is light. Gradually beat in the sugar and continue beating until light and fluffy. Change the mixer speed to medium. Beat in the eggs, one at a time, beating well after each addition, then beat in the sour cream, mashed bananas, and the vanilla.
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5
Change speed to low. Beat in the flour mixture, just until blended.
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6
Spoon half of the batter into the prepared pan. Scatter half of the walnut and sugar sprinkle over the batter. Thinly slice the third banana over the batter. Top with the remaining batter and scatter the remaining walnut and sugar sprinkle over the top.
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7
Bake until a skewer inserted halfway between the sides and the tube of the pan comes out clean, about 1 hour and 15 minutes. Let cool in the pan on a wire rack for 20 minutes. Carefully loosen the cake from the sides and tube of the pan, then invert onto a wire rack and turn right-side-up. Serve warm or cool, sprinkled with sifted confectioners' sugar, if desired.
