Put an oven rack in the center of the oven. Preheat the oven to 300 degrees F.
Put the white chocolate in a medium bowl. Set aside. In a small saucepan, over low heat, add the cream and milk and heat until hot but not boiling. Pour the cream mixture over the chocolate and stir until smooth. In another medium bowl, whisk the egg yolks, sugar, lemon zest, and salt together until smooth. Slowly whisk the cream mixture into the egg mixture until smooth. Ladle the custard into 4 (4-ounce) ramekins or custard cups. Arrange the ramekins in an 8 by 8-inch square baking dish. Pour enough hot water in the baking dish to come halfway up the sides of the ramekins. Bake for 30 minutes until the custard is almost set and still slightly soft in the center (custard will set as it cools). Let the ramekins cool for 30 minutes in the baking dish. Remove the ramekins from the water and refrigerate for 3 hours or up to 1 day.