Preheat oven to 350'F. Spray a 9x13" baking pan with cooking spray; set aside.
In a medium bowl, whisk together flour, baking powder and salt; set aside.
In a large bowl, mash bananas until they become liquefied. Add softened butter, and combine well; stir in sugar, eggs & vanilla. Mix well.
Add milk and flour mixture; stir until blended. Set aside and make the Crumb Topping.
Stir the brown sugar and flour together, cut in the cubed butter with a fork or pastry cutter, until a coarse crumb forms and will hold together when you press it in your hand. This takes a few minutes, but totally worth getting the topping blended well.
(Use this as a filler too, if you like a crunch in the middle of your cake. Just pour half the batter into the prepared pan. Top with 1/3 of the crumb mixture. Pour the remaining batter over the crumb mixture and top that with the rest of the crumbs, or just sprinkle all over the top, and bake.)
Bake at 350'F. for 45-50 minutes or until a toothpick comes out clean. Cool for 10 minutes.
Stir the confectioners' sugar and cocoa together, then add milk. Use a whisk to blend, so there won't be any lumps.
Drizzle glaze over top, and serve warm or cold! Store in an airtight container.