Preheat oven to 350F. Butter a 10" tube pan and dust with flour, tapping out excess and set aside.

Walnut-sugar Sprinkle: Combine the walnuts, sugar and cinnamon in a small bowl and set aside.

Sift together flour, baking powder and salt and set aside.

In a large bowl, cream the butter until light. Gradually beat in the sugar and continue beating until light. Add eggs, one at a time, beating well after each addition. Beat in Sour cream, mashed bananas and vanilla.

Add flour mixture until just blended. Spoon half batter into a prepared pan. Scatter half of the walnut sugar sprinkle over the batter. Thinly slice the third banana over the batter. Top with remaining batter and scatter the remaining walnut sugar sprinkle over the top.

Bake for 1 hour 15 minutes or until skewer is clean. Cool in the pan for 20 minutes. Remove cake from pan and sprinkle with sifted confectioner's sugar, if desired.