In a thick saucepan combine condensed milk, butter and cocoa powder. Place the pan on the stove over low heat. Start mixing with a rubber spatula until the mixture thickens significantly and starts pulling out of the pan. It will take about 30 minutes but depends on your stove and the pan.
Let the mixture cool for 6 hours at room temperature.
Grease your hands with some butter. Make walnut-size balls out of the mixture and coat them with the sprinkles.
Arrange the Brigadeiros in paper cases.
Notes
Store the candies at room temperature for up to 3 days. They keep refrigerated for up to a week.
If you want to make variations, you can flavour your Brigadeiros with spices or extracts. You can also coat them in other types of sprinkles or stuff them with fruits.