Whisk the sugar, egg yolks and lemon juice in a bowl for 1-2 minutes until fluffy. Then, add the cornstarch, flour, and vanilla, and mix until smooth.

In a saucepan, heat the milk over medium heat with the lemon peels (to infuse) until it begins to simmer (but not boil). Remove the lemon peels, then, gradually pour the hot milk into the egg yolk mixture while whisking constantly to temper the yolks.

Return the mixture to the saucepan and cook over medium heat, stirring continuously, until it thickens (about one minute after boiling). If the mixture is lumpy, continue whisking until smooth. Remove from heat and strain if needed.

Gradually add the softened butter in chunks, mixing until fully incorporated.

Cover the lemon pastry cream with plastic wrap and let it come to room temperature while preparing a 9x13 / 22x33cm baking dish and your Graham crackers.

Once the cream is cooled to room temperature, place very cold 36% fat-content heavy cream in a mixing bowl and use an electric hand mixer to whip the mixture for about 2-3 minutes until fluffy. Do not overwhip.

Gently loosen the lemon pastry cream with a few quick whisks, then carefully, using a rubber spatula, fold in the whipped heavy cream, ensuring the mixture stays light and airy.

To assemble, divide the graham crackers into 3 (as there will be three layers) and the diplomat cream into 2 (as there will be two layers).

Place one-third of the graham crackers in a single, tight layer at the bottom of a 9x13-inch baking pan. Break the crackers as necessary to fit. Evenly spread half of the lemon diplomat cream on top. Add a second layer of graham crackers on top of this. Then, spread the remaining lemon diplomat cream. Finally, top with a third layer of graham crackers.

Refrigerate while preparing the whipped cream layer.

Place very cold heavy cream with the sifted powdered sugar into a mixing bowl. Use an electric hand mixer to whip the mixture for about 2-3 minutes until fluffy.

Spread the whipped cream layer evenly over the graham cracker layer. Chill the cake for 4 hours before serving, for the layers to soften. Serve the Lemon icebox cake after 4 hours of chilling.

Decorate with lemon slices, fresh basil or mint, and a sprinkle of lemon zest.