Combine the unsalted butter, caster sugar and plain flour in a bowl and bring together into a dough using the egg white. Set aside the dough and chill in the fridge.
Mix the double cream, sugar and lemon peel, and gradually heat in a saucepan until it nearly reaches boiling point, then remove from the heat to cool.
When the mixture is cool add the lemon juice, then heat the mixture again and before it reaches boiling point pass through a sieve.
Pour into glass ramekins and leave to set in the fridge.
Roll out the dough and cut out round shapes, lay on a sheet of baking paper and bake at 180 C for 5 minutes.
Once cooled, add a biscuit to each of the lemon pots.