Preheat oven to 350 degrees F. Lightly grease a 8x8 inch pan.
In a large bowl, mix together flour, sugar, brown sugar, baking powder, baking soda and salt. Set aside.
In a medium bowl, mash the banana. Add sour cream, oil, egg and vanilla. Mix until well combined. Stir thoroughly into the flour mixture.
For the streusel:
In a small mixing bowl, stir all ingredients together until crumbly.
Spread half the cake batter evenly into the prepared pan. Sprinkle with 1/2 of the streusel. Pour the remaining batter evenly over the streusel. Top with remaining streusel.
Bake for 35-40 minutes, until a toothpick comes out clean.
For the glaze:
Whisk all ingredients in a medium bowl, until creamy and smooth. Drizzle over warm coffee cake. Slice and serve.