Add the condensed milk and cocoa powder. Cook on medium-high stirring constantly (so you don't but it) for 15-20 minutes, or until it's thick enough to stay together. If you drag your spatula through the mixture you should see the bottom of the pan for a bit before it fills back in the space.
Put a silpat on a baking sheet. Pour the mixture on a silpat and allow to cool for about 15 minutes.
Coat your hands in butter and create 1½ inch balls.