In a medium saucepan, add sugar, sifted cornstarch, and salt.
Slowly add lemon juice and cold water to the sugar mixture, stirring until smooth. If you notice any lumps of cornstarch, use a rubber spatula to press the lumps against the bottom or side of the saucepan to remove them.
Add the beaten egg yolks and butter to the saucepan and stir until the yolks are thoroughly blended with the lemon juice mixture. (The butter will remain, but it will melt into the mixture in the following step.)
Stirring constantly with a wooden spoon or heatproof spatula, gradually add boiling water to the saucepan. Continue to stir until all of the boiling water has been added and the mixture is cohesive and smooth.
Place the saucepan over medium heat and continue to stir the mixture gently until it comes to a gentle boil. The mixture will thicken as it heats.
When the mixture reaches a gentle boil, reduce the heat to medium-low/low and allow it to simmer slowly for one minute while you continue to stir it gently. At this point the curd should have the consistency of pudding, and it will thicken a bit more as it cools.
Remove the pan from the heat and immediately stir in grated lemon peel.
Transfer the lemon curd to a bowl (so that it does not continue to cook from the residual heat of the hot pan) and let it cool completely. I like to stir it a few times as it cools to avoid a skin forming on top of the curd, or you can place a piece of plastic wrap directly on the top of the curd as it cools to avoid a skin forming.