In a saucepan over medium heat, combine condensed milk, cocoa powder and butter.  

Stir with a wooden spoon until well combined. 

Continue stirring and cooking over heat until the mix thickens. Once it's ready, it will hold its shape for a few seconds when you separate it in the pan with the back of a spoon. 

Pour the mixture onto a greased plate and let cool in the refrigerator about 15-20 minutes. 

Once cooled, use a teaspoon or tablespoon depending on the size you want, to scoop out a small chunk. 

Grease your hands with butter, and roll the chocolate into a round ball. 

Drop the ball into the sprinkles, roll it around, and transfer to a mini muffin cup. Repeat until the mix is gone. 

Refrigerate to set and you are ready to serve.