Preheat the oven to 325°F. Place a 6-ounce ramekin in the center of a 9-inch cake pan.
The melted ice cream should be cool or barely warm. Whisk in the egg yolk until smooth.
Pour the mixture into the ramekin. Place the cake pan and ramekin together in the oven. Pour hot water into the cake pan to make a water bath. The water line should rise halfway up the ramekin.
Bake for 30-35 minutes, or until the edges are set and the center slightly jiggles. Cool the custard in the pan for 10 minutes and then remove the ramekin, dry it off, and allow it to set in the fridge for at least four hours, but ideally overnight.
Let the crème brûlée sit at room temperature 30 minutes before serving. Sprinkle the sugar over the surface and shake the ramekin to make an even layer. Use a kitchen blow torch to caramelize the sugar, or place the ramekin under the broiler for one or two minutes to do the same. Once the caramel layer cools, serve immediately.
