Preheat oven to 300F.
Prepare a bain marie (water bath) on the stove.
Once the water is boiling, place eggs, sugar, key lime juice and zest into a large bowl and begin whisking.
Whisk vigorously over the boiling bain marie until thick and light (approx. 5-10mins). Take on and off bain marie every once in a while to prevent setting/cooking eggs. Alternatively, try using a hand mixer. The mixture is ready when it's light, thick (like pudding), and ribbons form when you run your whisk through it.
Remove from heat and whisk in melted butter.
Pour into pre-baked tart crusts and bake for 5-7minutes until set. The center of the pies should jiggle just slightly the pan is nudged.
Cool completely on a wire rack, then place in fridge for at least 2 hours.
Serve cold, topped with whipped cream if desired.