Always measure out all of the ingredients before making the recipe. This is especially important to prep when you need ingredients like cold cubed butter and boiling water.
Also, ensure that you have all your equipment gathered. For this recipe, steps have to happen relatively quickly and the mixture shouldn't stand for too long.
In a medium saucepan, combine the granulated sugar, cornstarch, and fine salt. Add the boiling hot water and place the saucepan over medium heat.
Then, cook it until it begins to thicken. Make sure to whisk every couple of seconds to ensure the sugar melts and the mixture doesn't burn. You should also cook it until it becomes clear in color and not foggy.
Once the liquid is clear in color, you can add the lemon juice and lemon zest (if you want to add any). Give them a whisk to ensure they are well incorporated.
Finally, finish off the sauce by whisking in the cold cubed butter off of the heat. You will see the sauce starts to thicken and get a more solid consistency.
Once your sauce has been made, you should allow it to cool before adding it to your cheesecake.
If you are lucky enough to have any leftovers, or if you are making the lemon sauce for cheesecake in advance, then you can store it inside an airtight glass container. Place the sauce in the fridge where it can easily last up to 2 weeks.