• Before you begin, you will get the best results if you mash the bananas first then measure out the exact amount needed.
• Preheat the oven to 425°F (you will reduce the temperature after the muffins cook for 5 minutes)
• Grease 12 standard-sized muffin tins or line them with paper baking cups. In a medium bowl, whisk all the dry ingredients together then set aside.
• In another bowl, beat the butter and brown sugar then add the eggs and beat until well blended. Next add the vanilla, and mashed bananas and beat until well combined.
• Fold the wet mixture into the flour mixture then fold in the blueberries.

Make the cinnamon crumb topping:

• Blend the flour, sugar, and cinnamon together then stir in the melted butter until you have pea-sized clumps.
• Fill the greased muffin tins completely full with the batter then sprinkle the tops with the cinnamon crumb topping. For slightly smaller muffins, you can fill the tins ¾ full which will give you 18 muffins (but you will need to reduce the baking time by a couple minutes).
• Bake in the preheated oven at 425°F for 5 minutes then turn the oven down to 350°F and bake for 12 to15 more minutes or until a toothpick inserted in the center comes out clean.
• Allow the muffins to cool completely before removing from the muffin tins. The tender muffins can fall apart if you remove them while they are still warm.