Preheat oven to 350 degree Fahrenheit and lightly grease and flour a 9x9 baking pan. Set aside.

Combine the butter and sugar in the bowl of a stand mixer or using a hand mixer. Beat until well-combined, then stir in mashed bananas.

Add the eggs, mixing well after each addition, and then add sour cream and vanilla extract and stir well.

In a separate, medium-sized bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt.

Gradually add the dry ingredients to wet, stirring until completely combined (be sure to scrape down the sides and bottom of the bowl). I prefer to do this with a spatula to avoid over-mixing the batter.

Pour batter evenly into the prepared pan.

Melt butter in a medium-sized bowl on 10 second increments (stirring between) until just melted (you don't want it to be too hot).

Use a fork to whisk together the flour, brown sugar, granulated sugar, cinnamon, and salt. Pour melted butter over mixture and use a fork to stir until crumbly and combined.

Sprinkle streusel evenly over batter.

Bake on 350F for 35 minutes, or until centers spring back to the touch and a toothpick inserted in the center comes out with few moist crumbs or clean.

Allow to cool before drizzling with glaze (if using) and slicing and serving.

If desired, make glaze by whisking together powdered sugar, milk, and vanilla. Drizzle over cooled banana coffee cake.