In a medium pot, mix the sweetened condensed milk, cocoa powder, and butter.
Cook, over medium heat, stirring constantly. The mixture should start to thicken and get glossy. Heat until 93 C / 200 F, when tested with a candy thermometer. If you don't have a candy thermometer, you can cook the mixture until it is thick enough that when you scrape the bottom of the pot, the mixture will take 2-3 seconds before it flows back together.
Scrape the mixture into a shallow dish and chill in the fridge for an hour.
Once chilled, remove from fridge. Have a plate ready to place the brigadeiros once ready.
Place sprinkles in a small, shallow dish. Rub a small amount of oil over the palms of your hands.
With a small spoon, scrape up about 1 Tbsp of the chocolate mixture. Roll it between your palms until it is round. Place on the plate you have ready.
Once all the brigadeiros are rolled into balls, roll them one by one in the dish of sprinkles until they are covered.
Once brigadeiros are covered in sprinkles, place them in small paper muffin liners.
Store the brigadeiros in the fridge until you are ready to serve them.