In a small saucepan, bring the key lime juice to a soft boil. Reduce the heat to medium-low and simmer until the juice has been reduced to about 1 tbsp (about 3-5 minutes).
In another pan over medium heat, combine cream, zest, and sugar. Cook, stirring occasionally until steam rises (about 2-3 minutes). Remove the pan from heat and allow it to cool for 5 minutes.
Set a pot of water to boiling on the stove. Preheat oven to 300 degrees.
In a heatproof bowl, whisk egg yolks and vanilla until combined. Slowly drizzle in the cream mixture, whisking rapidly the whole time. Stir in lime juice. Strain mixture through a metal sieve set over a bowl and divide among ramekins.
Place a kitchen towel in the bottom of a deep dish (I use a lasagna pan). Place the ramekins in the pan and add boiling water halfway up the sides of the ramekins. Cover pans loosely with foil and bake until custards are set about 40-45 minutes.
Remove ramekins from pan and transfer to a wire rack. Cool to room temperature, and refrigerate for at least 3 hours.
Just prior to serving, sprinkle a tsp of sugar over the top of the custard. Using a kitchen torch, move the flame until the sugar melts and creates a hard top. Serve immediately.
Notes
This recipe is my go to for homemade graham crackers.This is based on a lemon creme brulee recipe I have had for over 17 years. I don't know the origin of it and thus can't give proper credit to the original author of it.I always use fresh key limes because I like the taste, but they do sell bottled key lime juice. If you plan to juice your own, keep in mind they're hard little buggers. I use this hand juicer and it works well.