First, prepare your lemon zest sugar. Wash and dry the lemons. Pour the sugar into a bowl, then zest 2 pink lemons directly into the bowl. You will want about 2 tablespoons of lemon zest. Then, with clean hands, press the lemon zest into the sugar, so that the sugar takes on the lemon flavor. It's okay if the sugar clumps up a little.
After the lemons have been zested, cut them in half and juice them, straining out the seeds and any pulp, until you have 1/2 cup of lemon juice. Because pink lemons are so small, you may need up to 6 lemons for this.
Then, separate the egg yolks from the whites. (You can save the egg whites in the fridge for another recipe!) Place the egg yolks in a saucepot, and add in a whole egg. Mix until the yolks and egg are incorporated - try to get rid of any pockets of egg whites. Leave them in the pot on the kitchen counter - do NOT put it on heat yet!!
Whisk in the sugar and lemon zest mixture with the yolks until they're evenly combined. Then, mix in the strained lemon juice and whisk until everything is smooth and evenly combined.
NOW put the pot on low heat on the stove. Whisk constantly until the mixture thickens enough to coat the back of a wooden spoon (see note). This may take up to 10-15 minutes. Be sure to constantly whisk so that the eggs don't set!!
Once the curd has thickened, remove it from the stove and add in the cubed butter. Whisk the butter into the mixture until totally melted, and the curd is smooth.
Finally, strain the curd through a wire, mesh strainer to remove any pieces of zest or egg that may be left over. This step is optional but recommended.
Once the curd has cooled slightly, put it in a glass jar in the refrigerator to chill and set for at least 1 hour.
Enjoy!