Prepare cake according to instructions found on the box, let it cool. Use a fork to fluff up the cake. Pour the sweetened condensed milk into the fluffed cake. Mix the sweetened condensed milk using your hands and then form the mixture into one inch balls. Freeze the cake balls for 30 minutes. Dip the frozen cake balls into melted chocolate, make sure all sides are coated. Place on sheet tray lined with wax paper and allow the chocolate to cool and harden. If adding sprinkles, make sure to add them before the chocolate coating has a chance to harden.