In a large bowl, beat the eggs, egg yolk and sugar with an electric mixer, until fluffy. Slowly add butter and beat for 1 min. Mix in the lemon juice. The mixture will look curdled, but it will smooth out as it cooks.

Sit the bowl over a pan of gently simmering water, making sure the water is not touching the bottom of the bowl. Leave to cook for 10-13 minutes, stirring every now and again, until the mixture is creamy and thick enough to coat the back of a spoon.

Remove the curd from the heat; stir in the lemon zest.

Transfer the curd to a bowl. Press plastic wrap on the surface of the lemon curd to keep a skin from forming and chill the curd in the refrigerator. The curd will thicken further as it cools. Covered tightly, it will keep in the refrigerator for a week and in the freezer for 2 months.