Use a 6-quart slow cooker and a heat-resistant dish that fits all the way inside your stoneware. I used a 1 1/2-quart casserole dish. Add water around the dish, until halfway up the side of the dish. (You are using the slow cooker as a bain-marie, or water bath.)
In a mixing bowl, whip together the eggs, cream, sugar, and vanilla. Pour the mixture into the dish, cover, and cook on high for 2 to 4 hours. The custard should be set with the center still a bit jiggly. Touch the surface lightly with your finger to check. Unlike an oven, it will be difficult to overcook this. Try not to worry.
Very carefully (use oven mitts!) remove the dish and let cool completely on the countertop, then chill in the refrigerator for 2 to 3 hours. Before serving, sprinkle sugar over the top and brown with a kitchen torch, or place under the broiler for 3 to 5 minutes, or until the sugar has browned.