First, mash the banana (a fork works well). Then cream together the mashed banana, butter, sugars, and vanilla extract until well combined. Then add the cinnamon, flour, baking powder, and salt, and stir until a soft dough ball forms. Then stir in the walnuts.
In a separate, smaller bowl, use a pastry blender or your clean hands to combine the crumble topping ingredients until they form, well, a crumbly topping.
Divide the dough into 12 balls (a cookie scoop works really well here). You can also use the back of the scoop to press a little indention in each dough ball before adding the crumble top. This little bowl indention helps to keep more of the crumble top on the cookie, rather than rolling off the sides.
Bake at 325°F for 15-16 minutes. Then, remove to a cooling rack.