In a saucepan over medium heat, stir the zest and juice with ½ the sugar, until the sugar has dissolved. Remove from heat.
In a heatproof bowl, whisk the remaining sugar with the eggs until well blended.
Gradually whisk the hot mixture into the eggs and sugar, whisking constantly until blended.
Pour the mixture back into the saucepan and heat over medium heat, whisking constantly until thickened, about 2-3 minutes.
Remove from heat and whisk in cubed butter, a couple of cubes at a time until smooth.
Pour into a bowl, place a piece of plastic wrap directly on the surface of the lemon curd, and refrigerate for at least 3 hours.
After 3 hours, preheat oven to 400℉.
Pierce bottoms of tart shells with a fork.
Bake tart shells in preheated oven for 10-12 minutes or until golden brown.
Remove from oven and allow to cool for 10 minutes.
Remove lemon curd from the fridge and gently stir.
Fill each cooled tart shell evenly with the lemon curd.
Place in fridge, covered, for 30 minutes before serving.
Top with whipped cream just before serving if desired.