Combine the lemon zest and juice with the butter and sugar in a small heatproof bowl. Rest the bowl over a small saucepan of gently simmering water, ensuring the bowl doesn’t touch the water. Turn the heat down to low and whisk until the butter has melted.
Whisk the egg and egg yolk together and add to the lemon mixture. Gently cook, whisking regularly, until the curd thickens. This should take about 15 minutes. Spoon into a cooled sterilized jar and seal with a lid. Use within 4 weeks. Refrigerate once open