Preheat the oven to 350 degrees.
For the crumb topping/filling, combine brown sugar and flour in a food processor and process until mixed.
Add in 1/2 cup cold butter and process until it forms pea sized crumbles.
Pour crumbs into a bowl and place in the refrigerator until needed.
In a small bowl, whisk together flour, baking powder, cinnamon, and salt and set aside.
In the same food processor you used before, combine bananas and lemon juice, mix until smooth.
Add in softened butter, sugar, egg, and vanilla.
Slowly pulse in flour mixture and milk until smooth.
Line a cupcake pan with cupcake liners and be ready to scoop the batter and crumb filling/topping.
Fill cupcake liners about half way full with batter and then add about a 1 Tablespoon layer of crumb filling.
Top the crumb filling with another layer of batter, almost to the top of the liners.
Add a heaping scoop or two of crumb filling/topping over the batter so that it makes a large mound.
Bake for 30 minutes or until the center is set and a toothpick comes out clean.
Let cool for 10-15 minutes before adding the glaze.
To make the glaze, combine powdered sugar and water and mix well.
Drizzle glaze over muffins.
Remove muffins from the pan and cool completely on a wire cooling rack.