Adapted from A Good Bake by Melissa WellerMaking marshmallows can be sticky business and non-stick spray is your best friend here. If you don't have it, pour some vegetable oil in a small bowl and wipe surfaces with a paper towel dipped in that. The original instructions said the spray the pan first, before lining it with foil, which I didn't think was necessary if you line the pan well. But you are welcome to spray the pan as well first. You will need a stand mixer to make this recipe. Perhaps a handheld mixer would work, but most aren't powerful enough to beat the thick marshmallow mixture to the right consistency. Be sure to use a saucepan that holds at least 4-quarts (4l) as the syrup will foam and bubble up while cooking. Following the recipe are some FAQs you might have about the recipe, so I've answered them there. And you didn't hear it from me, but if you're looking for a serving suggestion, these chocolate marshmallows are a special treat melting on top of a cup of Parisian hot chocolate...
1cup (240ml)cold water, divided
3packets (21g)unflavored powdered gelatin
2 1/3cups (465g)granulated sugar
1/2cup (160g)light corn syrup
1/2cup (45g), plus 3 tablespoons (25g)unsweetened cocoa powder, preferably Valrhona or another good-quality Dutch-process cocoa powder
Line the inside of an 8-inch (20cm) pan with foil so it goes up and over the edges, smoothing out any wrinkles with your hands as best you can. (The fewer the creases, the easier the marshmallows will be to release from the pan later. Spray the foil with non-stick spray.
Pour 1/2 cup of cold water into the bowl of a stand mixer. Sprinkle the gelation over the water and attach the whip attachment to the mixer. Let stand for at least 5 minutes, to allow the gelatin to absorb the water.
In a medium-to-large saucepan (at least 4-quarts/4l) stir together the sugar, corn syrup, 1/2 cup cocoa powder, honey, salt, and the remaining 1/2 cup water. Fit with a candy thermometer, if using a clip-type thermometer (rather than an instant-read one, which I used here), and cook over medium-high heat until the temperature reaches 240ºF (116ºCº).
Remove from heat and with the mixer on high speed, in a slow, steady stream, gradually pour the hot syrup into the gelatin, being careful to pour the syrup between the bowl and the whip, not directly on the whip or it'll splatter against the sides of the bowl and not mix with the gelatin. Don't rush this step; it'll take about 2 minutes to get it all incorporated.
Continue to whip the mixture on high speed until the marshmallow mixture is fluffy and the outside of the bowl feels room temperature to the touch, about 15 minutes. Whip in the vanilla extract.
Scrape the mixture into the prepared cake pan. Sift together the 3 tablespoons of cocoa powder and powdered sugar in a medium to large bowl, and sift a dusting of the mixture over the top of the marshmallows, just enough to cover it lightly. Let set, uncovered at room temperature, 8 to 10 hours, or overnight.
Unmold the marshmallow mixture onto a cutting board, flipping it so the exposed surface is down, and peel off the foil. You may need to coax it away with your fingers, but it's quite flexible and forgiving, so feel free to use your clean hands to help it release from the foil. Use a sharp chef's knife (or scissors) to cut the marshmallows into squares of whatever size you'd like, cleaning the blade with hot water and wiping it clean between every few cuts, to remove marshmallow build-up on the knife and ensure clean slices. Working in batches, toss the marshmallows as few at a time in the cocoa powder mix, put them in a mesh strainer to shake off any excess.
Storage: The marshmallows will keep for up to one week in an airtight container at room temperature.