Line 8x8 baking pan with parchment paper.
Combine powdered sugar, cornstarch, and cocoa powder in a small bowl.
Coat the lined pan with half of the powdered sugar mixture and set aside.
Place the gelatin and one cup of room temperature coffee in the bowl of a stand mixer with the whisk attached, and set aside.
In a medium saucepan, combine the other cup of coffee, sugar, and corn syrup.
Cover and heat over medium-high heat for about 5 minutes.
Remove the lid and attach a candy thermometer. Continue heating until the mixture reaches 240° F. Remove from heat.
Start the mixer on the lowest speed and slowly stream in the hot mixture.
Once the hot mixture is added, slowly (to avoid splatter!) increase the mixer speed until it's on the fastest speed.
Mix for about 10 minutes.
Pour marshmallow mixture into the prepared pan.
Sprinkle the remaining cocoa powder mixture over top of the marshmallow mixture.
Let set for at least 4 hours.
Cut into cubes, or use cookie cutters to cut into fun shapes.