In a pot, off the heat, whisk together the lemon juice, lemon zest and corn starch until combined.
Add in the eggs, butter and sugar. Using your whisk break up the eggs and whisk all the ingredients together until fully combined.
Place your pot over medium/low heat. You will want to whisk as the ingredients warm up to temperature together.
Turn the heat up to medium high. Continue to whisk as it comes up to boil. The boil is essential for the corn starch to thicken the curd. Once it starts to boil, keep cooking for another 30-40 seconds.
Over a clean bowl or pot, pour the lemon curd through a fine mesh strainer to remove the zest and any of the over cooked bits that from the bottom of the pot that were accidentally stirred in.
Cool it down in an ice bath. This step is important, otherwise you will risk the possibility of the lemon curd separating. Let it cool down completely in an ice bath.
To save: pour into a jar or container and cover with plastic wrap pressed down on top of the lemon curd to help prevent a film from building. Store in the refrigerator until you want to use it. Before use, give it a quick stir to loosen up.