In a double broiler, melt butter and add sugar then grate in 3 tbsp of lemon zest.
In a separate bowl crack 4 whole eggs and beat in 1 cup of lemon juice and 1 tsp of corn starch.
Whisk the egg mixture into the butter and sugar, make sure you whisk it continuously for about 8 minutes. When you see a few bubbles break the surface the lemon curd is done. Do not let it boil!
Strain out the zest and cool completely at room temperature and put plastic wrap directly on the surface of the curd to cool completely in the fridge before storing.