Preheat oven to 400 degrees F.
Prepare a muffin pan with 12 muffin liners.
In a large bowl, mash bananas until only small chunks remain. Add in melted (and cooled) butter and brown sugar and stir until well combined. Stir in beaten eggs and vanilla and stir until combined.
In a separate bowl combine flour, baking soda, baking powder, cinnamon and salt. Stir to combine.
Combine wet and dry mixture until just combined. Do not over-mix. Fold in blueberries, taking care to be gentle and not over mix, just enough that they are combined with the batter.
Divide batter between 12 muffin cups.
If adding optional topping, combine all topping ingredients in a small bowl then sprinkle over the muffin cups, dropping onto the batter.
Place in oven and immediately reduce the oven temperature to 350 degrees F.
Bake for approximately 20 minutes or until a toothpick inserted into the center of a muffin comes out clean, and tops are turning golden brown. Remove from oven and allow to cool for five minutes. Remove muffins from pan and serve.