Cream butter and sugars. Mix in egg and buttermilk. Mix in dry ingredients until just combined. Fold in chocolate chips. Cover bowl with towel and let rest for at least 1 hour.
Divide batter into prepared muffin tin, filling 3/4 full. Place the muffin tin in a 425° oven and immediately turn down to 350º. Bake for 20-22 minutes.