Place a strainer over a medium-sized bowl near the stove.
Place the sugar in a small bowl and zest the lemons onto the sugar. Use your fingers to rub the sugar and zest together. Juice the lemons until you have 3/4 cups of juice.
In a nonreactive 2-quart saucepan, combine the whole eggs, eggs yolks, and sugar and whisk until well blended. Stir in the lemon juice, butter, and salt.
Cook the mixture over medium heat, whisking constantly until the mixture is thickened. As you cook the mixture the butter will melt. When the mixture has thickened and reached a temperature of 185 degrees F pour it into the strainer. Press the curd through the strainer with a spatula to remove the solids.
Cover the surface of the curd with plastic wrap, gently pressing the plastic wrap onto the surface of the curd. Place the curd in the refrigerator and chill.
The lemon curd is good in the refrigerator for 1 week and can be frozen for up to 1 year. If freezing the curd, pack it into a freezer container after it has chilled. When ready to use place it in the refrigerator to thaw for 24 hours before intended use.