Spray a 9 X 13 inch baking dish with nonstick cooking spray and set aside.
In a medium bowl, combine flour, baking powder, baking soda and salt. Set aside.
In a liquid measuring cup combine buttermilk, lemon juice, and vanilla. Set aside.
In a large bowl add sugar and lemon zest. Using an electric mixer set on medium, beat until mixture is very fragrant 1-2 minutes. Transfer 1/4 cup of the sugar mixture to a small bowl and set it aside. Add butter to the remaining sugar mixture and beat until mixture is fluffy, about 2 minutes. Add the eggs, one at a time, mixing after each addition just until combined. Add the egg yolk and mix to combine. Set mixer to low speed and add flour mixture in 3 additions alternating with buttermilk mixture. Mix until ingredients are just blended.
- 2 1/2 Cups - Cake Flour
- 1 Teaspoon - Baking Powder
- 1/2 Teaspoon - Baking Soda
- 1/2 Teaspoon - Salt
- 3/4 Cup - Buttermilk, at Room Temperature
- 1/4 Cup - Fresh Lemon Juice
- 1 Teaspoon - Vanilla Extract
- 1 3/4 Cup - Sugar
- 3 Tablespoons - Fresh Lemon Zest
- 12 Tablespoons (1 1/2 Sticks) - Unsalted Butter, at Room Temperature
- 3 - Eggs, Large, at Room Temperature
- 1 - Egg Yolk, at Room Temperature
- 3 Cups - Powdered Sugar
- 3 Tablespoons - Fresh Lemon Juice
- 2 Tablespoon - Buttermilk