Put the lemon zest and juice, the sugar and the butter into a heatproof bowl. Sit the bowl over a pan of gently simmering water, making sure the water is not touching the bottom of the bowl. Stir the mixture every now and again until all of the butter has melted.
Lightly whisk the eggs and egg yolk and stir them into the lemon mixture. Whisk until all of the ingredients are well combined, then leave to cook for 10-13 minutes, stirring every now and again, until the mixture is creamy and thick enough to coat the back of a spoon.
Remove the lemon curd from the heat and set aside to cool, stirring occasionally as it cools. Once cooled, spoon the lemon curd into sterilised jars and seal. Keep in the fridge until ready to use.
