Preheat oven to 375 degrees F. Line a muffin tin with liners and set aside.
Whisk together the flour, sugar, baking soda, and salt in a medium bowl and set aside.
In a large bowl, mash the bananas with a fork until only small lumps remain. Mix in the egg, vegetable oil, vanilla, and walnuts.
Add half of the dry ingredients to the wet ingredients and mix just until combined. Add in the rest and mix just until combined.
Use a 1/4 cup measuring cup to measure the batter into the baking liners.
In a small bowl, mix together the topping ingredients and mix with a fork or your fingers until a fine crumble forms. Sprinkle the crumble over the top of the cupcakes and bake for 18 to 22 minutes.