Melt milk chocolate chips over simmering water in a double boiler, stirring frequently with a rubber spatula. When chocolate is completely melted, add whipping cream, stir together with a whisk. Remove from heat, stir in butter one tablespoon at a time, until completely melted. Cover and place in the refrigerator for several hours or until firm. (If you're in a hurry, place in the freezer for about 15 minutes, do not freeze.)
Cover a cookie sheet with wax paper. Using a teaspoon, shape the firm mixture into balls, place the candies on wax paper and return to the refrigerator for about an hour or until firm.
Roll the truffles in cocoa powder or confectioner's sugar and store in an airtight container in the refrigerator. Truffles are the best when served chilled.