Preheat the oven to 325ºF. In a small saucepan, heat together the whipping cream, the milk, the lime zest, and the sugar. When the mixture is very hot and steamy, pour it over the chopped white chocolate. Let the mixture stand for 1 minute, then carefully stir with a wooden spoon until you have a smooth, glossy, white chocolate sauce, then stir in the egg yolks.

Divide the mixture between 4 ramekins placed in a 9x9 brownie pan. Pour boiling water between the ramekins and the pan until the water comes halfway up the sides. Carefully transfer the pan to the preheated oven. Bake the pots de crème until they are set but still jiggle in the middle, about 45 minutes.

Carefully take the pan out of the oven, and take the ramekins out of the water bath. Set them on a cooling rack and let them cool completely before transferring them to the refrigerator to chill overnight. Serve the white chocolate pots de crème topped with a generous mound of watermelon granita