Preheat oven to 350°F (175°C) and grease the cake pan.
In a bowl, mash the bananas until smooth.
In another large bowl, cream butter and sugar until fluffy.
Beat in eggs one at a time, then stir in vanilla and mashed bananas.
Mix flour, baking powder, baking soda, and salt in a separate bowl.
Gradually add the dry ingredients to the wet mixture, alternating with sour cream.
Fold in the chocolate chips and pour batter into the prepared pan.
For the crumb topping, mix flour, brown sugar, and cold butter until crumbly. Sprinkle over the batter.
Bake for 35 minutes, or until a toothpick comes out clean.
Cool the cake for 10 minutes, then serve warm or at room temperature.