Preheat oven to 350F and lightly grease a 9x9 baking pan with baking spray (or grease and flour, or line with parchment paper). Set aside and prepare streusel.

Whisk together flour, sugars, cinnamon, salt, and nuts.

Drizzle melted butter over the mixture and use a fork to claw together until you have a clumpy streusel. Measure out chocolate chips and set aside along with streusel for later.

Combine butter and sugars in a large mixing bowl and use an electric mixer or stand mixer to beat until well creamed.

Add eggs and beat until well-combined. Stir in mashed banana and vanilla extract.

In a separate mixing bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.

Add half of the dry ingredients to the wet and use a spatula to gently fold together until nearly combined. Add buttermilk, stir until nearly combined, then add remaining flour mixture and stir until just combined.

Spread the batter evenly into prepared pan. Top batter with the streusel mixture in an even layer and sprinkle mini chocolate chips overtop, if using.

Transfer to center rack of oven and bake for 45-50 minutes or until a toothpick inserted in the center comes out clean. Allow to cool before slicing and serving.