In KitchenAid (or in medium-sized bowl with electric mixer), combine butter and condensed milk.
Stir in cake mix.
Gradually add in powdered sugar and flour, stirring until ingredients are combined.
Stir in sprinkles on low speed.
Pour batter into 8x8 or 9x9 pan, cover with clear wrap, and refrigerate for at least one hour.
Once batter has chilled, spoon into approximately 1 ½" sized balls (if truffles become too sticky to manage, return to refrigerator and allow to chill for another 30 minutes).
Refrigerate truffle balls in refrigerator while you prepare the chocolate coating.
Microwave chocolate chips in medium-sized microwave-safe bowl for 30 seconds and stir, then microwave at 15 second increments (pausing between each increment to stir) until all chips are melted.
(If you are going to be using white chocolate, melt this now, too, and set aside).
Allow chocolate coating to cool for several minutes and then carefully cloak truffles in chocolate (if you don't have specialty dipping tools, I've found that the easiest way to do this is to set the truffle on the blade of a butterknife and use a spoon to coat the truffle, then gently slide the coated truffles off of the butterknife onto a wax-paper lined cookie sheet).
Drizzle truffles with white chocolate (if using) and sprinkle with additional colored sprinkles.
Transfer to refrigerator and chill for at least 30 minutes before serving.