Preheat oven to 350*.Line muffin tins with paper liners and set aside.
In small bowl, whisk flour, baking powder, baking soda and salt.set aside.
In another small bowl, combine buttermilk, lemon zest and vanilla.set aside.
In a medium bowl, beat butter till light and fluffy, on high speed, beat in egg then egg yolk, lower speed to low, beat in flour mixture alternately with buttermilk mixture, just till blended.
spoon batter into prepared muffin tins.
Bake till tops are golden and springy.about 20-22 minutes.