Use a bowl for the curd that will rest in a small saucepan without touching the bottom of the pan. Boil water for the pan so that it will come part way up the side of the bowl when it is submerged but without over-flowing.
Beat together the egg, egg yolk, lemon juice and zest, and sugar in a small bowl. Add the chunks of butter.
Rest the bowl in the pan of hot water so that the water comes up the side without overflowing from the pan or into the bowl. Simmer over a low heat to keep the water hot. Stir the lemon-butter mixture regularly as the butter melts then constantly as the mixture starts to thicken.
It's ready when thick enough to stick to the back of a spoon, around 5min. Remove the lemon curd from the heat and transfer to a jar or other container. Strain if you think you may have any stray lumps of egg in the mixture otherwise it's not necessary. You can either use proper canning method, or cover the top of the curd with cling wrap/film until it cools slightly before removing the cling wrap and putting the lid on (it needs to be airtight or it will form a skin).
Notes
This makes a little over ½ cup (120ml) of lemon curd.