It's originally taken from Nightscotsman's recipe on eGullet (which is a variation of a Martha Stewart recipe). The ingredients are his, and the description and clarifications are Aberwacky's. For the full discussion go to http://forums.taunton.com/tp-cookstalk/messages?msg=25540.1
1/4 cup corn starch (Nightscotsman uses potato starch)
1/4 cup powdered sugar
3/4 cup water
2 teaspoons vanilla
4 packages gelatin
3 cups sugar
3/4 cup water
1 1/4 cups light corn syrup
1/4 teaspoon salt
Baking sheet or pan
Each of the two main ingredients (gelatin and sugar syrup) is prepared separately and then combined. The gelatin is mixed with water and the sugar syrup brought to a boil.
First, combine the cornstarch and powdered sugar. Since marshmallows are one of the stickiest substances known on earth, this will serve as your non-stick coating. You won’t need it for a while but it's best to get it done ahead of time. While you're at it, oil a pan (a baking sheet for skinny marshmallows or a baking dish for fat ones) and coat it with the powdered sugar. You'll use this to pour the marshmallow goo into a little while down the road. You can also line a pan with parchment and spray it with nonstick spray--this works very well, too.
Next, sprinkle the gelatin over the 3/4 cup of water in a large mixing bowl and stir in the vanilla.
The sugar syrup comes next: combine the sugar, corn syrup and the remaining 3/4 cup of water in a large sauce pan and heat it to dissolve the sugar. Grab a candy thermometer, attach it to the pan, turn up the heat and let the sugar boil without stirring until it reaches 235 degrees (soft ball stage).
When the thermometer reaches 235 degrees, take the pan off the heat and, with the beaters running pour it into the mixing bowl. Be careful--don't let any of the sugar syrup splash up and burn you. A stand mixer is very handy for this-use the splash guard if you have one.
Whip the marshmallow mixture on high for 10 minutes. It will thicken, turn glossy white and feel like marshmallow cream.
Now, remember that pan you coated with the powdered sugar? Find it and scrape the marshmallow goo into the pan, smoothing it with a oiled spatula. Let it sit for several hours or overnight until it feels springy, and turn it out onto waxed paper coated with more powdered sugar. Let it sit for a few more hours and then it will be ready to cut.
You can cut your marshmallows into whatever shapes you desire--strips, cubes even triangles. I like to use scissors to chop up the marshmallow hunk--it's easier than using a knife and the cuts are cleaner. Dust the cut edges with some of the powdered sugar/cornstarch mixture to keep them from sticking. You can also use a pizza cutter or a dough scraper. (Leigh note: I now use the pizza cutter most often.)
Store the marshmallows in an airtight container for a month or two.
Use 1/2 cup fruit puree and 1/2 cup water with the gelatin instead of 3/4 cup water and vanilla. Proceed as otherwise indicated
Dissolve 1/2 cup of cocoa in 1/2 cup boiling water in a separate bowl, let it cool. Soften gelatin in an additional 1/4 cup cold water in mixing bowl. Add cocoa mixture to mixing bowl and proceed with recipe as above. This will produce a marshmallow with a strong chocolate flavor, but somewhat denser than the strawberry version. To get a lighter texture as well as a lighter chocolate flavor, reduce cocoa to 1/4 cup.