Preheat oven to 350 degrees
Line three 8” round cake pans with nonstick spray, flour and parchment paper.
Add butter and sugar to bowl of electric mixer, cream using paddle for 3-4 minutes or until light and fluffy, stopping occasionally to scrap down the sides.
Add Greek yogurt, vanilla extract and lemon zest, then mix until combined.
Add eggs one at a time, followed by the single yolk. Scrape sides each time you stop to add next egg + yolk.
In a separate bowl, combine AP flour, cornstarch, baking powder, basking soda and salt. Whisk to combine then set aside.
In small bowl, combine milk and lemon juice. Slowly, add dry ingredients to butter mixture, alternating with milk/lemon juice. Mix until well combined, stopping to scrap down sides as needed.
Divide cake batter evenly between the three 8 prepared pans and bake for 22-23 minutes or until a toothpick inserted in the center comes out clean or with only a few crumbs.
Allow cakes to cool 3-4 minutes, then take a butter knife and run it around the outside edges of each cake then flip and remove from pans and allow to continue cooling on rack.
If you’re making the cake ahead, once the cake is almost fully cooled, wrap each layer in 2 layers of plastic wrap, then seal each one in a gallon freezer bag, add them to a large baking sheet and freeze. (I do this with all my cakes, allows me to make them in stages, helps trap the moisture and makes frosting them easier later!)