In a medium size bowl, add the sugar and orange zest. Using your clean fingers, massage the zest into the sugar. Doing this will release the oil from the orange zest.
Pour the mixture into a small saucepan and add the egg yolks, whole eggs, pinch of salt, orange juice, cornstarch and lemon juice. Whisk everything until well combined.
Place pan over medium heat and stir constantly so the mixture heats up evenly.
Continue to whisk until the mixture has thickened. The mixture has thickened when it coats the back of a spoon.
Turn off the heat and stir in the butter.
Pour the hot mixture through a strainer to remove any clumps and ensure the curd is smooth and lump free.
Place a plastic wrap on top of the curd to prevent a skin forming. Place in the fridge to chill for about 2 hours or until it is set.