Melt butter in a small saucepan then move to the side of the stove to keep warm – the butter should not be hot. Take a deep saucepan about 24 cm (9 ½ in) in diameter and fill with 4 cm (12 in) of hot water. Heat to a gentle boil.
Place eggs, yolk, zest and juice in a metal bowl about 30 cm (12 in) in diameter. Whisk well then place on top of the saucepan. Beat with a whisk or egg beater until mixture thickens, about 3–4 minutes. Take bowl off the heat, sit it on a damp tea towel to stop it sliding around and trickle in melted butter, whisking continually. Season.