Whisk together the eggs and sugar in a medium saucepan until smooth and combined.

Slowly pour in the fresh lemon juice while whisking continuously to ensure a smooth texture without lumps.

Place the saucepan over medium heat, stirring constantly to prevent sticking.

Once the mixture begins to bubble, reduce the heat and cook for about 5-10 minutes or until it thickens enough to coat the back of a spoon.

Remove from heat and stir in the butter cubes until melted. If using lemon zest, add it now for extra flavor.

Pour the curd through a fine mesh strainer into a clean container to remove any bits of cooked egg or zest. Let it cool before sealing with a lid.