Mix the sugar and finely grated lemon rind in a bowl.
Whisk the egg and yolk, combine it with the lemon juice, then put it in a sauce pan. Strain the lemon juice with a tea strainer. That will give a nice result.
Mix together the ingredients from Step 1 and 2, and add the butter when the sugar dissolves.
Stir for over 10 minutes while heating the sauce pan in a double boiler. If you heat it directly over the heat, it will become lumpy, so heat slowly and steadily over a double boiler.
It is ready once it becomes shiny and you can see the bottom of the sauce pan while stirring.
Sudachi (citrus sudachi) variation: To make sudachi curd, use 2 sudachi, 20 g granulated sugar, 20 to 30 g butter, and 1 small egg (or just 1 yolk).
Story Behind this Recipe
I received domestically grown lemons.
It's easy to double boil by filling a shallow frying pan with boiling water, then once it cools, turn on the heat. As noted in Step 4, if it heats too quickly, it will turn lumpy, so double boil it slowly and steadily.