Preheat oven to 350 degrees, grease a 13x9 inch baking dish.
Mash or puree bananas until there are no lumps left.
In a large mixing bowl, combine bananas, sugars, eggs, vanilla, oil, and sour cream until well blended.
Add baking powder, soda, salt and flour and stir until combined.
Pour into greased 13x9 baking dish and spread to level out.
In a separate bowl, combine sugar, flour, walnuts and salt for streusel topping. Cut in the butter with a fork until mixture resembles large crumbs. Sprinkle evenly over cake batter.
Bake at 350 degrees for 40-55 minutes, until toothpick inserted in center comes out clean.
Wait until completely cool to cut into 12 even pieces.