Prepare Lemon Pastry Cream: In a medium saucepan, whisk together granulated sugar, egg yolks, and freshly squeezed lemon juice until light and fluffy.

Gradually blend in cornstarch, all-purpose flour, and vanilla extract for a silky texture.

Heat whole milk with lemon peel in a separate pan until just boiling, then remove the peel.

Slowly temper the egg mixture with warm milk, then return to heat and cook until thickened, about 1 minute, stirring constantly.

Mix in Butter: Once the lemon pastry cream has thickened, remove from heat and gradually stir in unsalted butter until fully melted.

Cover the surface with plastic wrap to prevent a skin from forming and let the mixture cool to room temperature.

Whip Cream: In a separate bowl, pour very cold heavy cream and whip with an electric mixer on medium-high speed until soft peaks form.

Gently fold the cooled lemon pastry cream into the whipped cream using a rubber spatula.

Layer the Cake: Grab a 9x13-inch baking dish and start assembling your Lemon Icebox Cake.

Begin with a layer of graham crackers at the bottom and spread lemon diplomat cream over the crackers, repeating until multiple layers are formed.

Prepare Topping: Whip more heavy cream with powdered sugar until fluffy and spread over the top layer of graham crackers.

Chill: Cover with plastic wrap and refrigerate for at least 4 hours, allowing flavors to meld together.