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 - large eggs, at room temperature
1 - egg yolk, at room temperature
3 cups powdered sugar
3 tablespoons fresh lemon juice
2 tablespoon buttermilk
Preheat oven to 325 degrees. Lightly grease a 9 X 13 inch baking dish with cooking spray.
In a medium bowl, combine flour, baking powder, baking soda and salt. Set aside.
Combine buttermilk, lemon juice, and vanilla in a small bowl. Set aside.
Combine sugar and lemon zest in a large bowl and beat using an electric mixer set on medium, until mixture is very fragrant 1-2 minutes. Reserve 1/4 cup of the sugar mixture and set it aside.
Add butter to the remaining sugar mixture and beat until 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.
Pour batter into prepared baking dish and smooth out the top. Bake 25-30 minutes or until a toothpick inserted in the center comes out clean. Allow cake to cool in pan set on a wire baking rack for 10 minutes.
Meanwhile, prepare the glaze by whisking powdered sugar, lemon juice and buttermilk together until even and smooth. Spread glaze over warm cake then evenly sprinkle the reserved sugar lemon zest mixture over the top. Cool the cake completely, at least for 2 hours, before serving.