• Preheat the oven to 350°F (175°C). Line an 8x8-inch (20x20 cm) square baking pan with parchment paper, leaving some overhang so you can easily lift the bars out later.

Make the crust

• Add the graham crackers and sugar to a food processor and blend into fine crumbs. Pour in the melted butter and pulse until the mixture resembles damp sand. Alternatively, crush the crackers in a sealed bag with a rolling pin, then mix with the sugar and melted butter in a bowl. 144 g graham crackers, 12.5 g granulated sugar, 70 g butter
• Tip the crumbs into the prepared pan and press them firmly into an even layer using the bottom of a glass or measuring cup. Bake for 8 to 9 minutes, until lightly golden and fragrant. Remove from the oven and set aside while you make the filling.

Make the lemon filling

• In a large bowl, whisk together the sweetened condensed milk, egg yolks, lemon juice, lemon zest, and salt until smooth and well combined. The mixture will begin to thicken as the lemon juice reacts with the condensed milk. 2 x 395 g cans of sweetened condensed milk, 4 large egg yolks, 180 g fresh lemon juice, 1/2 Tbsp lemon zest, Pinch of salt
• Pour the filling over the warm graham cracker crust and spread it evenly with a spatula.
• Return the pan to the oven and bake for 18 to 22 minutes, until the edges are set and the center has a slight jiggle when you gently shake the pan. The filling shouldn't look liquid or slosh around, but it will still be soft and will firm up further as it cools.
• Let the bars cool to room temperature in the pan, then refrigerate for at least 2 hours, or until completely chilled and firm. For the neatest slices, I recommend chilling for 3 to 4 hours or overnight.
• Lift the chilled bars out of the pan using the parchment paper overhang. Cut into 16 squares with a sharp knife, wiping the blade clean between cuts for neat edges. Serve chilled.</