Melted chocolate takes many forms. As a silky-smooth ganache, it can fill pastries or frost a decadent torte. As a sauce, it can be drizzled over ice cream or fruit. Simply melting a bag of chocolate chips in a double boiler, however, won't make it pourable, and it won't stay melted for long. The addition of a couple of ingredients will transform your plain melted chocolate into a liquid dream.
Many recipes for melted chocolate, such as ganache, include heavy cream. Heating the cream and pouring it into the chocolate will melt it and make it malleable.
Mixing some oil into your chocolate as it is melting lends less richness to it than cream, but it also has the effect of making the chocolate pourable.
While cream and oil are some of the more commonly seen ingredients in chocolate sauce recipes, you'll also often find others. Some recipes include butter in tandem with cream, along with flavoring liqueurs such as amaretto, raspberry or orange. These ingredients also help keep your melted chocolate liquid.
Contact with moisture of any kind will cause your melting chocolate to seize, turning into a grainy, clumpy mess, so be sure to use very dry utensils and never pour just water into melted chocolate.