Preparing: Fire up the oven to 325° and grease that 9x13-inch square baking pan! The excitement begins.
Mixing Dry Ingredients: Whisk the flour, baking powder, baking soda, and salt together in a medium bowl, and set it aside. It's about to play a crucial role!
Creating the Flavor Burst: Combine buttermilk, lemon juice, and vanilla in a liquid measuring cup. Meanwhile, beat granulated sugar and lemon zest until they become moist and fragrant. Remember, zest adds personality to the cake!
The Creaming Process: Transfer 1/4 cup of sugar mixture to a small bowl, cover it, and set aside. Then, whip butter into the remaining sugar mixture until it's light and fluffy. Patience here pays off. Add eggs and yolk gradually; it's all about precision.
The Grand Fusion: Reduce speed to low. Add flour mixture, alternating with buttermilk. What you'll end up with is a smooth batter. Pour it into the prepared pan and smooth the top.
Baking Time: Wait and watch until the cake turns golden brown. It'll take 25 to 35 minutes. Then, cool it down for 10 minutes.
Glazing Glory: Whisk confectioners’ sugar, lemon juice, and buttermilk. Spread it over the warm cake and sprinkle the reserved sugar mixture. Let it cool for at least 2 hours. Yes, the waiting might be unbearable, but it's worth it.