Step 1

Preheat oven to 350 degrees F. Lightly grease a 8x8 inch pan.

Step 2

In a large bowl, mix together flour, sugar, brown sugar, baking powder, baking soda, and salt. Set aside.

Step 3

In a small mixing bowl, mix flour, brown sugar and cinnamon. Add the melted butter and mix until crumbled.

Step 4

In a medium bowl, mash the banana and mix in the sour cream, oil, egg, and vanilla. Add to the flour mixture and stir until combined.

Step 5

Spread half the batter evenly into the prepared pan. Sprinkle with 1/3 of the streusel. Pour the remaining batter evenly over the struesel. Top with remaining struesel and walnuts.

Step 6

Bake for 30-35 minutes until toothpick comes out clean.