Preheat the oven to 375F and line a 12-count muffin pan with muffin cups.

In a medium bowl, whisk together sifted flour, baking powder, baking soda, and salt until well combined. Then, set aside.

Next, in a separate bowl, combine the smashed bananas, brown butter, spices, sugar, vanilla extract and spices together until well combined. Mix until smooth. Then, add the flour mixture to the wet batter. Mix until combined, but it's okay to have a few lumps, as you don't want to over mix the batter. Set aside.

Make Crumb Topping. In a small bowl, mix brown sugar, flour, walnuts, salt, and cinnamon. Then, cut in the cold butter using a fork or pastry cutter until it resembles coarse crumbs. Set aside.

Next, use a scoop or spoon to fill each muffin cup about 4/5 full with the batter. Then, top each cup of batter with crumble topping.

Then, bake the muffins in the preheated oven for 18-22 minutes, or until a toothpick inserted into the center comes out clean.

Once done, remove the muffins from the oven and let them cool in the tin for about 5 minutes. Then, transfer them to a wire rack to cool completely and enjoy!