Combine the zest and juice of all the lemons, the caster sugar and butter in a medium-sized saucepan.
4 medium Lemons, 200 g Caster sugar, 50 g Unsalted butter
Place over medium-low heat and stir until the butter melts and the sugar dissolves (about 5 minutes).
Whisk three egg yolks and one whole egg together in a jug. Pour the whisked eggs into the saucepan and whisk to combine.
4 medium Eggs
Turn the heat up and whisk continuously until the lemon curd thickens and coats the back of a spoon (this can take 5-10 minutes).
Strain the lemon curd through a sieve into a bowl to remove the lemon zest. Leave for 10 minutes to cool and thicken up.