Heat oven to 350℉/177℃.
Spray three 6-inch pans with baking spray. Set aside.
In a medium bowl, whisk flour, baking powder, baking soda, salt, and lemon zest together. Set aside.
In the bowl of a stand mixer fitted with the flat beater or in a large bowl with a hand mixer, combine butter and sugar.
Beat on high speed until light and fluffy, about 3-4 minutes.
Add eggs and egg yolk, one at a time, beating well after each addition.
Once all eggs are added, continue beating until light and fluffy, about 3-4 minutes.
Add flour mixture alternately with buttermilk, beginning and ending with flour mixture.
Stir in lemon juice and vanilla.
Divide batter evenly among prepared pans.
Bake for 30 minutes, or until cake tests done.