Overheating chocolate even for few seconds will push chocolate far and harden immediately. Use good quality chocolate that is specifically used for baking i.e that can be melted. You also get candy melts (chocolate discs) that are specifically used for melting on amazon. Candy melts are easy to work with and melts beautifully. Do not use choco chips melts if you want to store it for long. Choco chips and coconut oil melts will turn chalky.