Preheat the oven to 350°F (175°C) and grease the cake pan.
Mash the bananas in a bowl until smooth.
In a large bowl, cream butter and sugar until light and fluffy.
Beat in eggs one at a time, then add vanilla and mashed bananas.
Combine flour, baking powder, baking soda, and salt in another bowl.
Gradually add dry ingredients to the wet mixture, alternating with sour cream.
Fold in chopped walnuts and pour the batter into the prepared pan.
For the crumb topping, mix flour, brown sugar, and cold butter until crumbly. Sprinkle it evenly over the batter.
Bake for 40 minutes, or until a toothpick inserted in the center comes out clean.
Cool for 10 minutes, slice, and serve with coffee or tea.