Heat oven to 200C/180C fan/400F/Gas 6.
Blitz biscuits in a food processor to fine crumbs. Set aside two tablespoons of the crumbs. Add the melted butter to the crumbs in the food processor and blitz for a few seconds.
Transfer the biscuit mix to tin, and use back of a spoon to press down firmly and evenly to the edges to give an even base.
Place egg yolks in a medium bowl. Whisk in the condensed milk. Slowly whisk in lemon juice until combined. Pour the mix on top of the biscuit base.
Place the egg whites in a clean, grease-free bowl and whisk to stiff peaks. Whisk in sugar, a tablespoon at a time, until the mix forms a thick, glossy meringue. Spoon the meringue on top of the lemony layer in the tin.
Sprinkle the reserved biscuit mix on top of the meringue and bake for about 25 mins until golden brown. Leave to cool in the tin for 30 mins, put in fridge.
When you’re ready to serve, release the cake from the tin, remove the baking paper and serve in slices.
110g unsalted butter, melted
5 eggs, separated
550g condensed milk
juice of five lemons
75g caster sugar
20cm round, loose-bottomed cake tin, greased, then lined with baking paper