In the bowl of a stand mixer, pour 1/2 cup cold water and sprinkle gelatin over it. Let it bloom for about 10 minutes.
In a medium saucepan, combine 3/4 cup water, granulated sugar, corn syrup, and salt. Stir to dissolve, then stop stirring. Attach a candy thermometer and cook until the syrup reaches 240°F (soft-ball stage).
Turn the mixer to low speed and carefully pour the hot syrup into the bloomed gelatin. Once combined, increase the mixer to high speed and beat for 10–12 minutes, until the mixture is thick, glossy, and tripled in volume.
Add cotton candy extract and your desired food coloring. Gently swirl the colors into the mixture to create a marbled effect if you like.
Grease and dust your baking pan with a mix of powdered sugar and cornstarch. Pour the marshmallow mixture into the pan, smooth the top with a greased spatula, and let sit uncovered for 4–6 hours or overnight.
Once set, dust a clean surface with the powdered sugar and cornstarch mixture. Turn the marshmallows out of the pan, cut into squares, and coat each piece fully.