Preheat oven to 350°F. Grease a 9x13-inch baking pan with nonstick spray. Set aside.
In a large bowl, with a whisk or hand mixer, beat the butter, sugar, and eggs together.
½ cup unsalted butter, 1 cup granulated sugar, 2 large eggs
Add the banana, buttermilk, and vanilla and mix well.
3 bananas, ¾ cup buttermilk, 2 teaspoons pure vanilla extract
Stir in the flour, baking powder, baking soda, and salt.
2 cups all-purpose flour, 2 teaspoons baking powder, 1 teaspoon baking soda, 1 teaspoon kosher salt
Fold in 1¼ cups of the chocolate chips.
1½ cups mini chocolate chips
Pour the batter into the prepared pan.
Sprinkle the remaining chocolate chips over the top.
Bake for 30 minutes, or until a toothpick comes out clean.
Allow to cool completely in the pan before cutting into squares.