Basically the milk chocolate has a lot more cacao fat (AKA: cocoa butter) and sugar, and has milk solids that are not in dark chocolate. The milk solids in most chocolate are obtained from powdered milk, as well, not liquid milk or cream as one might expect (though some chocolates are made from reduced milk). read more
Make sure the top has no water residue in it before placing the chocolate chips. A double boiler is a special pan that holds water in the bottom and has a second pan that sits on top. The water in the bottom pan should never touch the bottom of the top pan. read more