Line a muffin tin with muffin cases
Combine all the dry ingredients for the muffins to a large mixing bowl and mix well with a whisk.
Add the mashed banana, sugar, egg, vanilla, and butter to a second mixing bowl and mix well.
Add the banana mixture to the dry ingredients and mix well. Do not overmix.
Divide the batter and fill the muffin tins up two thirds.
Sprinkle the crunchy topping over the muffin batter.
Bake for 20 minutes or until a skewer comes out clean when inserted into the muffins.