Preheat the oven to 350°F (175°C) and grease a 9x13-inch baking pan.
Cream butter, granulated sugar, and brown sugar until fluffy. Add eggs, mashed bananas, and vanilla, mixing until smooth.
In a separate bowl, combine flour, baking soda, baking powder, salt, cinnamon, and nutmeg.
Gradually mix the dry ingredients into the wet mixture, alternating with sour cream. Fold in chopped nuts.
Pour batter into the prepared pan, spreading it evenly.
For the crumb topping, mix brown sugar, flour, and cinnamon. Cut in cold butter until crumbly, then stir in chopped nuts. Sprinkle on top.
Bake for 45 minutes or until a toothpick comes out clean. Cool before slicing.