1. Add the eggs together with the sugar, and lemon juice to a heatproof bowl and place on a pot with simmering water. Heat up the mixture while stirring constantly until it starts to thicken and looks more or less like a pudding – takes about 8-10 minutes. The mix should reach a temperature of about 167°F (75°C). Take off the heat and press through a fine mesh sieve instantly. Add the lemon zest and the butter in small pieces and mix until the butter has melted completely and you have a nice and smooth lemon curd. Cover with a piece of plastic wrap directly on the surface of the curd (to avoid the development of a skin) and let cool down. You can also fill it into glass jars directly and close them. The curd can be stored up to a week in the fridge.