Preheat oven to 350 degrees F (175 degrees C). Grease a 12 cup muffin pan.
In a large bowl, combine mashed bananas, oil, sugar, eggs, and vanilla. Beat until thoroughly combined.
In a separate bowl, mix together flour, baking soda, salt, and 1 teaspoon of cinnamon. Gradually add the dry ingredients to the wet ingredients, stirring until just combined.
In a small bowl, combine walnuts, brown sugar, 1 teaspoon of cinnamon, and melted butter. Mix until crumbly.
Fill each muffin cup with batter and top with crumb mixture.
Bake in preheated oven for 20-25 minutes or until a toothpick inserted into the center of a muffin comes out clean.
Allow to cool before serving.
Interesting Facts
Bananas are a great source of potassium, fiber, and Vitamin B6.
These muffins are a great way to use up over-ripe bananas.
Muffins can be stored in an air-tight container for up to 3 days.