To prepare the streusel, stir together the sugars, cinnamon, salt, and melted butter in a medium mixing bowl until well combined.
Stir in the flour. Set aside while you prepare the cake batter.
To make the cake:
Preheat oven to 350 degrees. Spray a 9x9 baking dish with non-stick spray.
Add the flour, baking soda, baking powder, cinnamon, and salt to a large mixing bowl and whisk to combine.
Add the sugars and butter to a medium mixing bowl and beat with a mixer until light and fluffy.
Mash the bananas until smooth with a fork. Add to the sugar and butter along with the egg and stir until just combined.
Add the banana mixture to the flour mixture and stir until just combined.
Spread cake batter into prepared pan.
Take the streusel topping in your hands and press together to form large crumbs. Top the cake with the streusel. It will seem like too much, but use it all for the best streusel experience.
Bake for 30 minutes or until a tester inserted in the middle comes out mostly clean.
Cool completely.
Sprinkle the top with powdered sugar or drizzle with vanilla glaze before serving.
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Notes
Texture: We really packed the bananas into this cake, making for a very moist and dense cake. This will not have a light and airy crumb like some cakes.Streusel: If you like big chunks of streusel, use your hand to squeeze the streusel together to form big clumps. For smaller, finer streusel just stir together and then sprinkle over the cake.Glaze: We like to top this coffee cake with a simple powdered sugar glaze. Whisk together 1 cup of powdered sugar with 2 tablespoons cream or milk and 1/2 teaspoon vanilla. Add additional powdered sugar to thicken or milk to thin the glaze to your liking. Drizzle over the baked cake.Storage: Cake will last at room temp for about 2 days. Store in the fridge for up to 5-6 days.