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.
With a stand mixer, beat sugar and lemon zest until smooth. Transfer ¼ cup sugar mixture to small bowl, cover and set aside.
Add butter to remaining sugar in mixer and beat until light and fluffy. Beat in eggs and egg yolk one at a time. On low speed, add flour mixture in 3 additions, alternating with 2 additions of the buttermilk mixture. Beat until smooth.
Pour batter into a greased and floured 10x15 pan and smooth the top with a spatula. Bake at 325, for 25-35 minutes, or until golden brown. Transfer to wire rack and let cool 10 minutes.
For the glaze, whisk powdered sugar, lemon juice and milk until smooth. Gently spread over warm cake and sprinkle reserved sugar mixture evenly over the top. Serve warm or cool for at least 2 hours.