Preheat oven to 325 degrees.
Heat the heavy cream over medium heat until it starts to get bubbles along the edges.
Whisk together the egg yolks and granulated sugar.
Slowly pour about ¼ of the cream into the egg mixture while whisking constantly. Then, slowly pour in the rest of the cream while whisking constantly, until combined.
Strain the custard through a fine mesh sieve, a strainer or cheesecloth.
Whisk in the key lime juice
Place 2 ramekins or mini cocottes into a roasting pan and divide the custard between them.
Carefully pour enough hot water into the roasting pan until it comes halfway up the sides of the ramekins.
Bake at 325 degrees for 40-45 minutes, until the creme brûlée is set but still trembles in the center.
Place the creme brûlée in the fridge to cool for 2 hours (or up to 3 days, covered).
Before serving, remove from the fridge and sprinkle with grated key lime zest for added flavor.
Sprinkle each creme brûlée with 1-2 teaspoons of granulated sugar. Tilt the mini cocottes to evenly spread the sugar over the top of the key lime custard and pour off the extra sugar
Caramelize the sugar with a kitchen torch slowly moving it back and forth about 3-4 inches above the sugar.
*Note: If you do not have a kitchen torch, caramelize the sugar under the broiler for 2-3 minutes, watching carefully so that it doesn’t burn!