Preheat the oven to 350 degrees F. Spray a 9 inch square baking dish with nonstick spray.
In a large bowl, use an electric mixer to combine the sugar and butter. Mix in the eggs, one at a time. Mix in sour cream and banana. Add the flour, baking powder, baking soda and vanilla; mix until combined. Scrape the batter into the prepared pan.
Add all of the streusel ingredients to a medium bowl. Cut the butter into the dry ingredients until the mixture is crumbled. Sprinkle the streusel over the batter.
Bake 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.