The simple answer is to use chocolate candy melts instead of pure chocolate. Once candy melts harden, they stay that way until warmed at a high temperature in the microwave. Candy melts are also a little easier to melt than chocolate and stay melted for a longer time, as they made just for this purpose. White chocolate and milk chocolate chips work well too, but you'll need to be more careful not to burn them when melting. Once chocolate chips burn or harden, they cannot be melted again.