Preheat the oven to 325F. Spray a 9x13" baking pan with non-stick baking spray and set it aside. In a medium bowl, whisk together the flour, cornstarch, baking powder, baking soda and salt, then set that aside. In a small bowl or a measuring cup, whisk together the lemon juice, yogurt and vanilla extract and set it aside.
In a large bowl, use either a stand mixer fitted with a paddle attachment or a hand mixer to beat the sugar and lemon zest together on medium speed for about a minute, until the sugar has moistened and is fragrant. Then beat in the butter for 2-3 minutes, until the color lightens and it becomes fluffy. Add the eggs and egg yolk one at a time, beating between each addition. Scrape the sides of the bowl as needed.
On low speed, add in 1/3 of the flour mixture, mix until mostly combined, then mix in 1/2 of the yogurt mixture. Then another 1/3 of flour mixture mixed in, followed by the rest of the yogurt mixture. End with mixing in the remaining flour mixture until it's all just incorporated. Then transfer the batter to the pan and smooth it out.
Bake for 23-30 minutes, or until a toothpick inserted into the center comes out clean. While its baking, make the glaze. In a medium bowl, whisk together the confectioners sugar and lemon juice until smooth. It should be thick, but with a pourable consistency. Set it aside until the cake is done baking.
As soon as the cake is finished and you take it out of the oven, pour the glaze evenly over the hot cake and smooth it out gently. Then allow the cake to cool in the pan on a wire rack until cooled and the glaze has set (about a hour). Then slice it into squares with a sharp knife (for clean cuts, wipe the knife in between cuts). Store leftover cake tightly covered at room temperature.
