In a medium nonstick saucepan, combine the Sweetened Condensed Milk and the Unsweetened Cocoa Powder and stir until smooth so there are no lumps.
Add a knob of Unsalted Butter to the pan and place over low to medium-low heat.
Cook the mixture, stirring constantly with a wooden spoon or silicone spatula, scraping the bottom and sides, until it thickens and you can see the pan bottom when you drag the spoon through (about 10–15 minutes depending on heat).
Remove the pan from the heat and let the mixture cool slightly, then transfer it to a greased plate or bowl and chill until firm enough to handle.
Butter your hands lightly (use a bit more butter) and scoop small portions of the chilled mixture, rolling them between your palms into bite-sized balls.
Roll each ball in the Chocolate Sprinkles to coat, then place the finished brigadeiros in paper candy cups.
Refrigerate for at least 30 minutes to set, then bring to room temperature briefly before serving for the best fudgy texture.
