• 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.

Streusel

• Whisk together flour, sugars, cinnamon, salt, and nuts.
  1 ¼ cup (156 g) all-purpose flour, ½ cup (100 g) light brown sugar, ⅓ cup (67 g) granulated sugar, 1 ½ teaspoons ground cinnamon, ¼ teaspoon salt, ⅓ cup (38 g) chopped pecans or walnuts
• Drizzle melted butter over 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.
  6 Tablespoons (85 g) unsalted butter, ⅓ cup (65 g) mini chocolate chips

Cake

• Combine butter and sugars in a large mixing bowl and use an electric mixer or stand mixer to beat until well creamed.
  ½ cup (113 g) unsalted butter, ½ cup (100 g) granulated sugar, ¼ cup (50 g) light brown sugar
• Add eggs and beat until well-combined. Stir in mashed banana and vanilla extract.
  2 large eggs, 1 cup (235) mashed overripe banana, 1 teaspoon vanilla extract
• In a separate mixing bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.
  1 ¾ cups (220 g) all-purpose flour, 1 teaspoon baking powder, ¾ teaspoon baking soda, ¼ teaspoon ground cinnamon, ½ teaspoon salt
• 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.
  ¼ cup (60 ml) buttermilk

Assembly

• 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.