4 large egg yolks
1 large egg white
1 cup fresh lemon juice
Zest of 2 lemons
2 cups sugar
1 cup (2 sticks) unsalted butter, cut into pieces
Combine all of the ingredients in the top of a double boiler and whisk until smooth. Bring about 2 inches of water to a boil in the bottom of the double boiler. Put the top of the double boiler over it (the water should just barely touch the bottom of the upper pan), turn the heat down to low, and cook, whisking constantly, until the curd has thickened enough to coat the back of a wooden spoon, about 15 minutes. (The curd will thicken further as it cools.)
Remover the top of the double boiler. Pour the hot water out of the bottom pan and fill with cold water. Set the top of the double boiler back on the pan (the water should just touch the bottom of the upper pan) and set aside for about 30 minutes to cool.
Pour the curd into three sterilized half-pint jars, screw on the lids and let cool to room temperature. Then refrigerate for up to 2 weeks.