That depends on if you would like to temper the chocolate or not. Chocolate chips don't work as well for tempering the chocolate (the process of heating and cooling the chocolate to a specific temperature so that it's smooth and shiny). Honestly, if you are just making these treats to enjoy with your family, I think you can just use melted chocolate chips and skip tempering. The chocolate may not be as shiny, but it is going to melt immediately as soon as you add the steamed milk. If you are making these to give as gifts, you may want to go ahead and use chocolate bars and temper the chocolate. If you temper the chocolate, the result will be prettier - the chocolate will have a smooth and shiny surface.