Grease an 8-inch (20 cm) square baking dish with vegetable oil. Set aside.
1 tablespoon vegetable oil
Add the cold water to the bowl of a stand mixer bowl. Sprinkle the gelatin powder over the water. Attach the bowl to the mixer and insert the whisk attachment.
½ cup cold water, 3 tablespoons powdered gelatin
Combine the granulated sugar, corn syrup, cocoa powder, and salt in a large heavy-bottomed pot. Stir well. Heat over medium heat on the stovetop. Once the mixture begins to bubble, stop stirring. Cook until a candy thermometer reads 240°F (115°C).
1 ½ cups granulated sugar, 1 cup light corn syrup, ½ cup cocoa powder, ½ teaspoon salt
Set the mixer to low speed. Carefully pour the hot chocolate syrup into the gelatin mixture while the mixer is running. Add the vanilla extract. Continue to mix until the gelatin has dissolved. Increase to high speed and whip for 10 minutes until the mixture is thick, glossy, and has quadrupled in volume.
1 ½ teaspoons vanilla extract
Spoon the marshmallow mixture into the greased baking dish. Smooth the top with a butter knife. Let set for at least 4 hours at room temperature.
Sift half the powdered sugar onto a work surface. Dip a sharp knife in hot tap water, then dry it with a tea towel. Run the knife around the edges* of the marshmallow, then turn it over onto the sugared surface. Sprinkle the top with the remaining powdered sugar. Quickly flip the marshmallow right side up.
½ cup powdered sugar
Before slicing, dip the knife in hot water and dry it each time. Cut into approximately 25 squares.
Notes
* You may have to repeat the process a few times to release the marshmallow. You can use cocoa powder to dust your finished marshmallows instead of powdered sugar if you prefer. Replace the half cup of powdered sugar with the same amount of cocoa powder.Storage: keep marshmallows in an airtight container at room temperature for up to 3 weeks. Avoid humidity and heat, as they can cause the marshmallows to melt or become sticky. If stacking marshmallows, place a sheet of parchment paper between layers to prevent sticking.