Place 4 ramekins in baking pan. Carefully pour the custard into each ramekin, dividing equally. Open the oven and pull out the middle rack. Place the baking pan on the rack, then carefully pour the boiling water into the pan so that it surrounds the ramekins. The water should come up to about one inch from the top of the ramekins. Bake in the preheated oven until the custard is set in the center, for about 20 minutes.