Line an 8×8 inch (20×20 cm) square pan with parchment paper, leaving extra on the sides to lift the slice out easily later.

In a bowl, mix together the crushed Marie biscuits and melted butter until the mixture resembles wet sand. Press it firmly into the bottom of the pan using the back of a spoon. Refrigerate while preparing the filling.

In a clean bowl, whisk together the sweetened condensed milk, lemon juice, and lemon zest until combined; the mixture will thicken.

Pour the lemon mixture over the chilled biscuit base and smooth the top. If you like extra crunch or flavor, sprinkle with more crushed biscuits or desiccated coconut.

Refrigerate for at least 4 hours, or overnight for best results. Once firm, lift out of the pan and slice into 12 neat bars.