In a medium size saucepan, beat together 4 eggs and ½ cup granulated sugar.
Add ½ cup lemon juice, 1 tbsp lemon zest, and ¼ tsp salt, then beat again.
Place the saucepan over low/medium heat and cook about 5 minutes or until it thickens to the desired consistency. You need to whisk it all the time while cooking.
Turn off the heat and add ½ cup cubed butter and stir it in until the butter is completely melted and incorporated.
Strain the curd through a fine mesh sieve into a bowl and let it cool off. Then transfer to a jar and refrigerate.
