Combine heavy cream and corn syrup. Microwave on high for 70-90 seconds until hot.
Pour over dark chocolate. Let sit undisturbed for 60 seconds.
Stir slowly until ganache forms.
Refrigerate ganache for 10-15 minutes until it is solid enough to scoop into balls.
If hand rolling: scoop ganache into 1" balls using a cookie scoop or spoon. Spoon cocoa overtop and roll cocoa into ganache until a ball forms.
If using the rice ball mold: scoop ganache into 1" balls using a cookie scoop or spoon. Spoon cocoa into each cavity of the rice ball mold. Place ganache ball into cavity and spoon additional cocoa overtop. Close mold and shake vigorously.