Combine the lemon zest, lemon juice, butter and sugar in a small saucepan and bring it to a low boil over medium low heat. Let the butter melt and the sugar dissolve while you give it an occasional stir. While the lemon mixture heats up, beat the egg yolks thoroughly together in their own bowl until they are liquid.

Take the lemon mixture off of the heat once it starts to gently boil, then pour it through a fine mesh strainer to get rid of the lemon zest and any little particles in the lemon juice. Take a 1/4 cup of the lemon mixture and whisk it into the egg yolks to temper them. Transfer the remaining lemon mixture back into the sauce pan and heat it back up over medium low heat. Pour the tempered eggs into the sauce pan while you whisk it into the lemon mixture. Keep whisking for about 2 minutes to let the lemon curd thicken. The mixture can bubble around the edges but do not let it come to a boil or else the eggs will scramble.

Once it has thickened for 2 minutes, take it off of the heat and let it cool for a couple of minutes. Then spoon it into a clean container. Cover the lemon curd with plastic wrap right on top of it so that it doesn't develop a skin, then seal the container with its top. Let the lemon curd chill for at least 3 hours before using it in countless ways!