Finely zest the lemons into a small mixing bowl using a microplane grater.
Juice zested lemons using a citrus juicer; you’ll probably need about 2-3 large juicy lemons to give you ⅓ cup of juice.
Place lemon juice, egg yolk and sugar in a heatproof bowl over a pan of simmering water (double boiler) and cook the curd over medium-high heat whisking constantly.
After about 8-10 minutes, the mixture starts to thicken like a custard (at this point the curd will reach around 80C /176F).
Remove it from heat, and pour the cooked curd through a sieve.
With a rubber spatula, mix in room temperature butter in small chunks one by one, making sure each chunk is fully incorporated before adding the next. Hand blend, if desired.
Fold the zest in at this point.
Pour curd into a jar and place plastic wrap directly on top so it is touching the top of the curd. (This prevents skin forming on top.) The curd will continue to thicken as it cools in the fridge.
Store refrigerated for about a week or freeze for up to 3 months
Notes
Don't set the heat too high under the double boiler, and make sure you whisk the mixture continuously to prevent the egg yolks from scrambling. All the ingredients need to be at room temperature before you begin to emulsify properly. Butter takes around one hour out of the fridge to soften correctly. If you use cold butter, it will not melt into the curd.