• Preheat oven to 350F and lightly grease and flour a 9x9 baking pan. Set aside.
• Combine butter and sugar in the bowl of a stand mixer (or in a large bowl and use an electric mixer). Beat until well-combined, then stir in mashed bananas.
• Add eggs, stirring well after each addition, and then add sour cream and vanilla extract and stir well.
• In a separate, medium-sized bowl, whisk together your 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 prepared 9x9 pan. Prepare Streusel.

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 your flour, sugars, and cinnamon. Pour melted butter over mixture and use a fork to stir until combined (mixture should be crumbly).
• 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 and milk. Drizzle over cooled banana coffee cake.