Preheat oven to 375 degrees. Line muffin pan(s) with paper cups. Set aside.
In a separate bowl, whisk together the egg, egg yolk, vanilla, buttermilk, and oil. Add the wet ingredients to the dry ingredients and mix until incorporated.
Fill muffin cups ½ full. Do not overfill! About 13 to 14 muffins depending on the size of the cups. Set aside.