1. Grease large shallow pan or platter with butter. In 4-quart nonstick heavy saucepan, add dulce de leche, cocoa and 2 tablespoons butter to boiling, stirring constantly with wooden spoon.
2. Reduce heat to medium-low; cook 10 to 15 minutes, stirring constantly, until mixture is thick and shiny and starts to pull away from bottom and side of saucepan when pan is tilted. Pour mixture into shallow pan; cool completely (refrigerate to firm up faster).
3. Place remaining ingredients in separate small bowls. Use teaspoon or melon baller to scoop truffle mixture, then use greased hands to shape into 1- to 1 1/2-inch balls.
4. Roll each ball in coconut, chocolate candy sprinkles, or colored candy sprinkles. Place in mini paper baking cups.
5. Store tightly covered at room temperature up to 2 days or in the refrigerator up to 1 week.