Cut cold butter into small cubes and set aside.

In a medium sized bowl add brown sugar, flour and cinnamon. Then add chopped butter and use a pastry blender or a fork to combine all ingredients until coarse crumbs form.  Add chopped walnuts and stir to combine.  Set aside crumb mixture while you prepare the cake batter.

Preheat oven to 350°F.

Lightly butter, then dust with flour a 9”x13” baking dish (or spray with cooking spray).

In a medium bowl, whisk together flour, baking powder and salt. Set aside.

In a large bowl (or the bowl of an electric mixer), beat butter until smooth. Add sugar and continue to blend.  Add very ripe bananas and blend until well combined.

While mixing, add eggs, then vanilla, milk and sour cream.

Add flour mixer and stir until just combined.

Pour ½ of the cake batter into the prepared pan, and then add a layer of the crumb topping (using a bit less than half of the crumb topping).

Pour the additional cake batter on top (using an offset spatula to spread as needed), then top with the remaining crumb topping.

Bake in preheated oven for 50-55 minutes.