Combine sweetened condensed milk, cocoa powder and butter in medium saucepan over medium-low heat.
Cook, stirring frequently, until mixture is very thick and a rubber spatula holds its line accross bottom of pan, about 25 minutes. Pour into a greased 8x8" baking dish and refrigerate until cooled, at least 30 minutes.
Working with approximately 1 tablespoon pieces at a time, roll into 1-inch balls. Place small sprinkles, chopped nuts, cocoa powder, etc. in small bowls, and roll chocolate in coatings until covered. If the candies stick to hands while rolling, spray them (hands) lightly with cooking spray.
The thickened chocolate mixture can be chilled up to 24 hours. The finished candies can be stored in the refrigerator up to 3 days.