Preheat oven to 350º F. Grease a 9" x 13" baking dish with butter or cooking spray; set aside.
In a large bowl, use your hands or two forks to crumble together the cold butter cubes, flour, sugars, and cinnamon until the mixture resembles a crumbly topping. Set aside the crumb mixture.
In a large bowl, whisk together flour, baking powder, cinnamon, and salt. Set aside.
In a stand mixer, mix softened butter on medium until creamy, about 2 minutes. Add the sugar and mix for another minute, until well incorporated. Then add the eggs, vanilla, and mashed bananas, and mix just until ingredients are incorporated (do not overmix!)
Turn the mixer down to low, and alternate mixing in milk and flour until all ingredients are combined.
Pour half of the cake mixture into the prepared baking dish. Top with ⅓ - ¼ of the crumb mixture. Then pour the remaining cake mixture on top of that layer and finally, top evenly with the remaining crumb topping.
Bake at 350º for 50-55 minutes, until a toothpick comes out clean.
To make the optional glaze: In a small bowl, whisk together the powdered sugar and 1 tablespoon of milk. Add more milk as needed until you get the glaze consistency you prefer. Set aside.
Cool at room temperature for 5-10 minutes before drizzling the glaze over the top, slicing, and serving.
Make a day ahead and store in an airtight container. Freeze in a freezer-safe container for up to one month. Thaw completely at room temperature before serving. DAIRY-FREE OPTIONS: Use vegan butter and almond milk if you want to keep this recipe dairy-free. Remember the butter must be COLD and the butter sticks should be cut into small cubes for the crumb topping. Do not attempt to use a spread of any sort for this.
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