• 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.

Streusel

• 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.

Glaze

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