Using a vegetable peeler, remove the colored zest from the lemons, leaving the white pith and add the zest to your sauce pan.
Combine the lemon zest, lemon juice, sugar, and eggs in a small saucepan on medium heat, whisking constantly.
Bring the mixture to a simmer, whisking the entire time for at least 10 minutes.
Place the diced butter in a medium bowl.
Place the pan over medium heat and add in the butter (or vegan butter). Stir until the butter has melted. Keep stirring until the curd is thick enough to coat back of a wooden spoon. Take care not to boil and to whisk very quickly; 5-7 minutes.
Spoon out the curd over a fine-mesh sieve into a glass bowl. Be sure to spoon it out a small portion at a time into the sieve. Place plastic wrap over the surface of the curd let the lemon curd cool slightly.
If using for a lemon tart, spoon into tart shell. Leave in refrigerator to cool down if you are using for another recipe.