In a medium saucepan over medium heat, combine the pure cocoa powder and the sweetened condensed milk, mix well with a whisk and add the unsalted butter.
Be sure to keep stirring and cooking the mixture over medium heat for 15 to 18 minutes, until thickened. do NOT let the brigadeiro burn.
Turn off the stove and if you wish, add the pieces of pure chocolate. Mix well for 2 minutes until it's completely melted.
Carefully, transfer or spread the brigadeiro mixture on a plate, cover with plastic wrap and let rest until cool enough to handle.
Put a little of the unsalted butter on your hands and hand roll the mixture into small little smooth balls.
Roll the finished balls in a topping of your choice. My favorites are chocolate sprinkles, shaved chocolate bits, pistachios, toasted almonds, frozen dry fruits and cookie crumbs.
Enjoy with your family and friends!