Begin by separating the egg yolks from the whites, which can be stored for future use, such as making Swiss meringue buttercream.
Next, zest the lemons. For maximum flavor, pulse the lemon zest and sugar together in a food processor to release the essential oils, though this step is optional.
Juice the lemons until you have just under 1/2 cup of juice. Be sure to strain out any seeds.
Strain the egg yolks into a saucepan and gently whisk them until smooth. Add the sugar, whisking thoroughly until the mixture becomes lighter in color. Gradually incorporate the lemon juice while continuing to stir.
Place the saucepan over low heat, stirring constantly with a whisk until the mixture thickens, begins to bubble slightly, and coats the back of a wooden spoon.
Remove the pan from the heat, and immediately add the cold, cubed butter. Stir until the butter has completely melted.
For an extra smooth texture, strain the curd to remove any zest or other small particles. The result will be a silky, luxurious lemon curd.
Transfer the finished curd to a sealable container or a bowl. If using a bowl, press plastic wrap directly onto the surface of the curd to prevent a skin from forming.