Wash your lemon then zest using a zester (personally I use a Microplane).Mix the zest with the sugar in a bowl that can fit on a saucepan. Add the egg while beating for a few minutes, until the mixture is frothy. Squeeze the lemon to obtain 50 ml of juice and pass the juice through a strainer to obtain a very smooth juice without pulp or seeds. Add the lemon juice to the sugar-egg-zest mixture and whisk again.
Heat the preparation in a bain-marie, stirring constantly with a whisk for 10 to 15 minutes, until the cream thickens. When the cream holds together, it's ready.
Off the heat, add the butter cut into small cubes and mix with a whisk until the butter is completely incorporated.
Transfer the lemon curd to a glass jar, film in contact then close the lid. Leave to cool then keep refrigerated.
