Preheat oven to 350°F. Line a muffin tin with 12 liners, or coat liberally with nonstick spray. Set aside.

Crumb Topping

1. In a medium bowl combine all the crumb ingredients. Mix until it form a dry, thick mixture. As the butter continues to cool, you’ll be able to break this into crumbs easily. Set aside.

Muffins

1. In the bowl of your stand mixer fitted with the paddle attachment, mix the butter and sugar together on medium speed for 1 minute. Add in the egg, vanilla, salt, and baking soda, and continue mixing until combined and smooth, scraping the sides of the bowl as necessary.
2. Turn mixer to low speed and add in both flours, mixing until just combined. Fold in the mashed banana until incorporated. Do NOT over-mix.
3. Fill the muffin liners 3/4 of the way through.
4. Break the crumb mixture into pieces with your hands. Evenly divide the crumb mixture on top of all the muffins. You can use all the crumb topping, or as much as you desire.
5. Bake the muffins for 20-25 minutes, or until set and a toothpick inserted in center comes out clean.
6. Allow the muffins to cool in the muffin tin for 10 minutes, and then transfer to a wire rack to cool.
7. Serve warm or at room temperature.