Preheat oven to 350 degrees. Prepare an 8 x 8 baking pan.
In a medium bowl, combine the flour, oats, baking soda, and salt. Set aside.
In a large bowl, cream the butter and brown sugar on medium speed of an electric mixer for approximately 3 minutes. Add eggs one at a time until well combined. Add bananas and vanilla.
Add the flour mixture, beating until well combined. Pour into prepared pan and set aside.
For the topping, in a medium bowl combine the rolled oats, brown sugar, melted butter, and cinnamon together until crumbly. Generously sprinkle the cake with the topping mixture.
Bake for 35 to 40 minutes or until a cake tester inserted into the center of the cake comes out clean. Cool for 20 to 30 minutes.
