Preheat oven to 350°F (175°C). Lightly butter an 8-inch square baking dish and line with parchment paper with 2-inch overhang on two sides.

Combine graham cracker crumbs, sugar, and melted butter until texture resembles wet sand. Press into pan, 1 inch up sides.

Bake crust 8–12 minutes until lightly golden. Cool completely before adding filling.

Whisk together egg yolks, sweetened condensed milk, lemon juice, and optional zest until smooth. Pour over cooled crust.

Bake for 15 minutes until filling is set. Let cool 30 minutes, then chill in fridge at least 1 hour.

Lift slab using parchment, cut into 16 squares with serrated knife, wiping between cuts. Serve chilled, optionally dusted with powdered sugar or garnished with berries.