Preheat oven to 375 degrees.
Use muffin liners or grease a standard 12-sized muffin tin.
For Brown Sugar Crumb: In a mixer, add 1/2 cup flour, 1/2 cup brown sugar, 1/4 teaspoon salt, and mix until well combined. Add 1/4 cup cold butter and mix until crumbly and well combined. Place in a small bowl and set aside.
For Banana Muffins: Mash 2 of the bananas and set aside.
Mix together remaining butter and brown sugar until light and fluffy.
Add remaining 3 bananas, mashed and mix until well combined.
Scrape sides of bowl and add eggs, vanilla, baking powder, baking soda and salt. Mix until well combined.
Add mashed bananas and flour, mix until combined.
Fill prepared muffin tin 3/4 of the way full. Divide crumb topping evenly over muffins.
Bake for 20-25 minutes until a toothpick inserted into the center of muffins comes out clean.