Place the sugar and lemon zest in a medium size heatproof bowl.
In a smaller bowl vigorously whisk the lemon juice together with the eggs. (It is important that the lemon juice and eggs are thoroughly combined to avoid ending up with flecks of cooked egg white in the curd.) Once combined, pour the mixture over the sugar and zest and mix.
Cut the butter into small cubes and add to the mix.
Place the bowl over a pan of barely simmering water (make sure the bowl does not touch the water) and whisk until thickened – approx. 15 mins. Do not be concerned if this takes a little longer
Notes
Keep your lemon curd fresh by storing it in a jar or airtight container in the refrigerator for 1-2 weeks. If you want to extend its shelf life, freeze the lemon curd for a month or more, and ensure to thaw it in the fridge before use.Optional additions:I like to add a pinch of flaky salt and a tsp of vanilla.