Mix the zest and sugar. In a medium bowl, combine the lemon zest and sugar. Set aside for 30 minutes to allow the flavors to infuse.
Prepare the double boiler. Fill a saucepan with about 1 inch of water and place it over medium-low heat. Allow the water to come to a gentle simmer. Then, place a heatproof bowl or the top of a double boiler over the simmering water, making sure the bottom of the bowl doesn’t touch the water.
Combine the ingredients. Whisk together the egg yolks and whole eggs in the heatproof bowl until well blended. Then, add the lemon juice and the sugar-zest mixture. Continue stirring, then slowly add the butter, one tablespoon at a time, allowing each piece to melt before adding the next.
Cook to 170°F. Stir the mixture constantly over the simmering water until it thickens and reaches 170°F on a thermometer. Using the thermometer is important, as the texture won’t visibly change much during cooking.
Strain (optional). Remove the bowl from the heat. If you like a smoother curd, use a fine mesh strainer to remove the zest and egg solids. This step is optional for those who like more texture.
Cool and store. Stir the curd for about 5 minutes as it cools, which will help it thicken further. Pour the lemon curd into sterilized half-pint mason jars, leaving about ½ inch of space at the top if freezing.