Preheat oven to 425°F, and grease a large 12 muffin tin. In a small bowl, sift together the dry ingredients, and mix the chocolate chips.
In a large bowl, cream together the butter with the sugar until light.
Whisk in the eggs, and vanilla extract until fluffy.
Whisk in the banana, and buttermilk.
Fold in the dry ingredients until just combined.
Divide among the muffin tins, place muffins in the oven, reduce the heat to 350°F, and bake for 20-25 minutes until a toothpick inserted into the center comes out with moist crumbs.
Cool in the tins for 5 minutes before removing, and completely cooling on a wire rack.