Start by zesting your lemons. You only want the yellow part of the skin. If you grate too deep and get the white part, your curd will taste bitter.
After you have the zest, squeeze the lemons until you have half a cup of juice.
Starting with a cold pan helps prevent the eggs from cooking too quickly the moment they touch the metal. Put your sugar, egg yolks, whole egg, and lemon juice into a small pot. Do not turn on the heat yet.
Whisk these items together until the color is even and the sugar starts to dissolve.
Turn your stove to a medium-low setting. Use a whisk or a flat spatula to stir the mixture constantly. You want to reach every corner of the pot so nothing sticks to the bottom. Keep stirring for about 8 to 10 minutes.
You will see the liquid start to change from a thin juice into a thick sauce.
You know the curd is done when it can coat the back of a spoon. Dip a spoon into the pot and draw a line through the coating with your finger. If the line stays clear and the liquid does not run, it is thick enough.
Take the pot off the heat. Drop in your cold butter cubes. Stir them in slowly until they melt and the sauce looks shiny.
