Combine the cold butter, flour, brown sugar and cinnamon in a large bowl and use a pastry cutter or sturdy fork to cut the butter into the dry ingredients, creating a crumbly topping. Set aside.
For the Banana Cake:
Preheat oven to 350 degrees F. Coat a 9x13" baking dish with non-stick spray and set aside.
Mash the bananas in a large bowl until smooth. Add in the melted butter, sugar, eggs and vanilla extract and whisk until combined.
In a separate bowl, whisk together the flour, baking powder and salt. Add half of the dry ingredients to the banana mixture, whisk to incorporate. Add in half of the milk, whisking again to incorporate. Whisk in the remaining dry ingredients followed by the remainder of the milk.
Pour half of the cake mixture into the prepared baking dish. Sprinkle the top with 1/3 of the crumb mixture. Pour the remaining cake batter over the top, followed by the remaining crumb topping.
Bake for 45 to 55 minutes, or until toothpick inserted in center of cake comes out clean.
Let cool slightly before adding glaze.
For the Glaze:
Combine the powdered sugar, milk and vanilla in a small dish and whisk until smooth. Drizzle over warm cake. Enjoy!