Preheat your oven to 375 degrees. Line a 12-count muffin tin with muffin/cupcake liners.
In a mixing bowl, mash the bananas with a fork. Add the brown sugar, vegetable oil, egg, and vanilla extract and mix until well combined.
Add the flour and then sprinkle the cinnamon, salt, baking soda, and baking powder over the flour. Stir everything together until just combined (try not to over mix).
Fold in the blueberries.
To make the streusel topping: add all topping ingredients to a bowl (butter, flour, brown sugar, cinnamon, and salt) and combine with a fork or your fingers until it resembles a course crumb.
Divide the muffin batter evenly in the muffin tin and sprinkle evenly with the streusel topping.
Bake for 16-18 minutes or until a toothpick inserted into the middle of a muffin comes out clean.