You will need a metal, plastic, or glass rectangular baking pan or mold that measures approximately 13 by 9 inches (33 by 22 centimeters). Pour the vegetable oil into the pan and use a piece of paper toweling to carefully grease the bottom and all the sides. Make sure that all inside surfaces are well greased, including the corners of the mold.

Pour the gelatin powder into a large glass mixing bowl. Bring the water to boiling. Pour the water into the bowls and stir gently with a rubber spatula until all the gelatin has dissolved. Note: You are using here only one cup of water, much less than what is called for on the package of gelatin. The powder will take longer than to dissolve and be thicker than usual.

Add the marshmallows to the bowl and microwave for about 45 seconds, until they look shiny and slightly puffy. (Time may vary, depending on your microwave oven’s voltage; keep an eye on this and err on the side of caution if you doubt.) We do not want the marshmallows to dissolve completely (in which case we would not get a separate fluffy layer in our spirals); we just want them warmed and puffy.

Take the bowl out of the microwave. Whisk the gelatin and marshmallows together until the marshmallows have dissolved and the mixture is light and fluffy.

Pour the mixture into the greased pan and refrigerate for a couple of hours. As it cools, the marshmallow will rise to the top, forming a separate, very light-colored, layer.

Once your jello is completely set, carefully run a knife or metal spatula around the edge of the pan, separating the edges of the gelatin from the mold. Working slowly and calmly with your fingers, remove the entire sheet of gelled dessert from the pan.

Place your gelatin sheet on a flat surface. Starting again from one of the shorter sides, roll the gelatin up into a firm coil.

Set the roll seam-side down. Use a piece of sewing thread or dental floss to cut the gelatin into slices about half an inch thick. Place the slices directly onto a serving platter, onto squares of waxed paper, or into paper cupcake liners to make them easy to grab.

Serve immediately or refrigerate until serving time. Any uneaten spirals will keep well, covered and refrigerated, for several days.