Preheat the oven to 350 degrees. Coat an 8 x 8 baking dish with cooking spray.
Make the brown butter streusel by heating the butter in a saucepan over medium heat, stirring often, until melted. It will start to bubble, foam and separate then will turn golden brown with a nutty aroma. Remove from the heat immediately and pour into a dish to cool. Side Note: Watch the butter carefully while cooking because it can burn quickly & easily.
Meanwhile, whisk together the flour, sugar, brown sugar, cinnamon and salt until well combined.
Add the cooled brown butter and mix with a fork until completely incorporated.
Add the ice water and mix until crumbly. Chill in the refrigerator until needed.
Make the banana coffee cake by mixing together the flour, white sugar, brown sugar, baking powder, baking soda and salt until well combined.
In another bowl, mash the banana very well then mix in the sour cream, oil, egg and vanilla extract until well combined.
Add the flour and mix until just combined.
Spread half of the batter evenly into the prepared baking pan.
Sprinkle with half of the chilled streusel.
Carefully spoon the remaining batter evenly over the top of the streusel and gently smooth. Sprinkle the remaining the streusel over the top.
Bake for 32-37 minutes, or until a tester inserted in the center comes out clean. Remove from the oven and allow to cool on a wire rack for a couple of hours.
Make the glaze by whisking the powdered sugar, heavy cream and vanilla extract until creamy and smooth. Drizzle the glaze on top of the cooled banana coffee cake then slice and serve. Enjoy.
Store remaining coffee cake covered in the refrigerator.