Beat 8 egg yolks in a bowl to break them up, then pour the yolks through a sieve into a heavy-bottomed pot to strain out the white membranes.
Stir in 1 ¼ cups sugar and ¾ cups lemon juice (3-4 lemons) and cook over medium heat, stirring constantly with a wooden spoon in a figure-eight motion.
Continue cooking until the curd thickens enough to coat the back of the spoon. Lift the spoon out of the curd and run your finger down the length of the spoon. If your finger leaves a line, the curd is thick enough. If the curd runs and covers up the line, it’s too runny and needs to cook longer.
Remove the pan from the heat and add 10 tablespoons of unsalted butter and the lemon zest.