In a shallow sauce pan or skillet, combine the condensed milk, cocoa powder, butter, and salt.
Heat this mixture over medium-low, stirring constantly.
Continue to stir and scrape the bottom of the pan to mix the chocolate together. You're done when scraping your spatula across the bottom of the pan leaves a gap that takes 2-3 seconds for the chocolate to flow in and fill.
Pour the hot mixture into a shallow bowl or a rimmed plate to allow it to cool. Place in the fridge for about two hours.
Once chilled, form the balls. I found it easiest to use a spoon and lightly grease my hands with butter to roll them out.
Pour your sprinkles of choice into a bowl and roll the brigadeiros to coat. Set aside, re-chill if you like, or serve immediately.