In a large bowl, whisk together flour, cornstarch, baking soda, baking powder, salt and sugar. Make a well in the center of the dry ingredients and add in the buttermilk, eggs and vanilla extract.
Whisk wet ingredients until blended, then whisk all ingredients together. Whisk vigorously until batter is smooth, then whisk in melted butter until it’s completely incorporated. Set the batter aside to rest for 15 minutes.