1. Prep the pan.
Heat the oven to 350°F (175°C). Grease an 8-inch square pan with a bit of butter, then line it with parchment paper, leaving some overhang on two sides to help lift out the bars later.

2. Make the crust.
Break up the graham crackers and blitz them in a food processor until they’re fine crumbs. Add the sugar and melted butter, then pulse again until everything sticks together like wet sand. Press this mixture firmly into the bottom of the prepared pan, pushing it up the sides about an inch to form an even crust.

3. Bake the base.
Place the crust in the oven for 10–12 minutes, until lightly golden. Let it cool completely before adding the filling.

4. Whip up the filling.
In a bowl, whisk together the sweetened condensed milk and egg yolks until silky smooth (a hand whisk or electric mixer both work). Add the lemon juice and mix until the filling is completely blended.

5. Assemble and bake again.
Pour the lemon mixture over the cooled crust, spreading it evenly. Return the pan to the oven and bake for another 15–20 minutes, until the filling is set and the crust edges are a light brown.

6. Cool and cut.
Let the pan cool on a wire rack until room temperature. For extra firmness, chill in the refrigerator for 30 minutes or longer. Once set, lift the slab out of the pan with the parchment paper and transfer to a cutting board. Slice into 16 smaller squares for bite-size treats, or 9 larger squares if you want heartier portions.