Preheat oven to 325° and grease and flour a 13x9 pan. Or line the pan with parchment and grease.
In a large bowl, combine cake flour (or all purpose and cornstarch), baking soda, making powder, and salt together. Mix and set aside.
In a separate bowl, mix zest and sugar together until moist and fragrant, about 1 minute. Transfer 1/4 cup of that mixture to a small bowl and set aside for later.
In a liquid measuring cup, combine buttermilk, lemon juice, and vanilla.
Add butter to remaining sugar mixture and beat until light and fluffy. About 2 minutes.
Beat in eggs and yolk, one at a time, until incorporated.
Add flour mixture 1/3 at a time, alternating with buttermilk mixture until all is incorporated and smooth.
Pour mixture into prepared pan and bake for 35-40 min (mine took 39).
Let cool for 10 minutes. I set a timer for this so I wouldn't forget.
While cake is cooling mix glaze ingredients together.
Gently spread over warm cake and sprinkle with reserved lemon sugar.
Let cool for at least 2 hours before serving.