Whisk together cornstarch and cocoa powder in mixing bowl.
Heat 1/3 cup coconut milk in microwave until warm (not steaming or boiling). This should only take about 20 seconds - adults should handle this step. Be sure to use a microwave-safe bowl and handle with care.
Pour warm coconut milk into mixing bowl with cornstarch and cocoa powder. Milk together with a spoon until a smooth slime begins to form.
Finish kneading by hand, making sure slime is cool enough to handle.