Makes 1¼ cups.
Zest and then juice your lemons. Whisk the whole eggs and egg yolks in a medium bowl and set aside. Measure out the sugar, salt and butter.
In a medium saucepan, add the lemon juice, zest, sugar and the salt. Over a low flame, heat while stirring until the sugar has melted and it is just on the verge of coming to a boil. About 4 to 5 minutes. Remove from heat.
Grab a whisk. Holding the pan with one hand, slowly drizzle about half of the hot lemon/ sugar mixture into the bowl of eggs, while rapidly whisking the eggs with the other hand. Now take the warmed egg/lemon mixture and whisk it back into the pan. This is called tempering the eggs.
Heat over a low flame while continuously stirring with a wooden spoon. Try to keep it below a boil and take care not to let the bottom of the pan catch. The lemon curd will thicken in about 6 to 8 minutes. It's done when it can easily coat the back of a spoon. It will continue to thicken as it cools.
Take the lemon curd off the heat and stir in one or two pats of butter at a time until it is melted. When all the butter has been melted and the curd is smooth, strain the curd with a fine mesh strainer into a bowl. Straining the curd is optional but it does remove any possible solids that may be there.
Take a piece of plastic wrap and press down directly on top of the curd to cover. This prevents a "skin" from forming as it cools. Refrigerate the lemon curd for 2 to 3 hours until it is cold. It is now ready for use.