Because they use modernist ingredients, these amounts are given in metric by weight. For more information on how to measure modernist ingredients check out this article.
Attach the whisk attachment to your standing mixer. Add the ingredients for the gelatin base and let the gelatin hydrate for 5 to 10 minutes.
Prepare a 33cm x 23cm (13" x 9") baking pan by spraying it with Pam. Combine the cornstarch and powdered sugar and sift some of the cornstarch mixture in the pan.
While the gelatin is blooming, combine the ingredients for the syrup in a sauce pan set over medium-high to high heat. Stir it lightly until the sugar is dissolved and then leave it alone while it heats. Cook the syrup until the temperature reaches 110°C to 116°C (230°F to 240°F).
Remove the syrup from the heat and pour it into the standing mixer on top of the bloomed gelatin and water. Turn the mixer on low and slowly increase the speed until it is on high, being careful not to splash the hot syrup out of the mixer.
Whip the marshmallow mixture until it has tripled in volume, it should take 8 to 15 minutes. When the marshmallows have almost been fully whipped, add the vanilla extract and cocoa powder and make sure it is fully incorporated.
Pour the marshmallow mixture into the pan and spread it out evenly. Sift the top with some of the cornstarch mixture. Let the marshmallow set for several hours, or preferably cover with cling wrap and let sit overnight.
Dust a cutting board with the cornstarch mixture. Turn the marshmallows out onto the cutting board and cut into the shapes you want.
Blend the cherry juice and agar together until well combined. Pour into a pot and bring to a boil while stirring occasionally. Let simmer for 3 to 5 minutes, stirring regularly.
Reduce the heat slightly to just below a simmer and keep it there while dipping the marshmallows.
Apply the cherry coating to the marshmallows using a pastry or BBQ brush. To help the coating harden more quickly, you can place the marshmallows in the refrigerator for 10 minutes before and after applying it.
The coating should begin to harden quickly. Once the coating has set, you can quickly re-dip it if you want a thicker coating.
Top each marshmallow with a cherry half and some dark chocolate then serve.