Preheat the oven to 350F.
Grease a 9x13 baking pan. Set aside.
In a large bowl, with a whisk or electric mixer, beat together the butter, sugar and eggs.
Add the banana, buttermilk and vanilla and mix well.
Stir in the flour, baking powder, baking soda and salt.
Fold in 1 ¼ cups of the chocolate chips.
Pour the batter into the prepared pan.
Sprinkle the remaining chocolate chips over the top.
Bake for 30 minutes, until a toothpick comes out clean.
Allow to cool completely in the pan before cutting into squares.