Clean/Steralise your Jam Jars so that the Curd will be able to last for its full time... I often just dishwasher clean them on their own so that they're squeaky clean!
Put the Zest & Juice of the Lemons, the Butter, and the Sugar into a heatproof bowl - I use a large glass bowl. Set the bowl over a pan of simmering water (not boiling water!!) and stir the mixture occasionally till the butter has melted!
Beat the Eggs & Egg Yolk together in a separate bowl so they're combined well, and then pour into the lemon mix - whisk all of the ingredients together so that they are combined well and whisk continually in the heatproof bowl for 10-14 minutes.
You will know when its done when the mixture will coat the back of a spoon without dripping off too much - it will have thickened considerably and be nice and creamy like custard - it will thicken more when cooling in the jar so don't be worried if its still pourable!
Set the Lemon Curd on the side to cool down completely - stir it occasionally as its cooling so that the consistency stays throughout, but it must be completely cool when poured into the jars. Once cooled, store in the sterilised jars and keep in the fridge! The Lemon Curd will last in the fridge, in sterilised jars, for two weeks!
To sterilise jars, wash the jars in very hot, soapy water or put through the hot cycle of a dishwasher. Place the jars onto a baking tray and slide into an oven set to 160C/325F/Gas 3 for 10-15 minutes - tip from the BBC Food website.
Alternatively, add all the ingredients into a large microwaveable bowl, and whisk together. Microwave on full power for minute intervals, whisking in-between. It'll be done when it thickens up and coats the back of the spoon as well.
If in a squeaky clean jar, it will last in the fridge for up to 2 weeks.