Preheat oven to 375 F and prepare a 12 count cupcake pan by greasing it or line with 12 cupcake liners.
In a large mixing bowl, combine the dry ingredients: all purpose flour, baking powder, baking soda, salt, and cinnamon. Whisk until combined.
In another bowl, combine the wet ingredients: egg, brown sugar, vanilla extract, mashed bananas and oil.
Transfer the wet ingredients to the dry ingredients mix and gently mix until fully incorporated. Do not overmix.
Using a scoop or a tablespoon transfer the banana muffins batter to the prepared cupcake pan.
In a small bowl combine the brown sugar, all purpose flour and cinnamon. Cut in the butter.
Crumble this streusel topping over the muffin batter.
Bake at 375 F for 20-25 minutes or until an inserted toothpick comes out clean.
Combine the vanilla icing ingredients and mix well. Drizzle over prepared banana muffins.
