Break the chocolate into pieces and add it to a saucepan along with the butter and condensed milk.
Heat over medium-low heat, stirring constantly until everything has melted and is starting to thicken.
Make sure you are continually scraping the bottom of the pan to avoid burning. I like to use a flat wooden spatula, but any heatproof utensil will do. Keep cooking until you can draw a line in the middle of the mixture, and it takes about 2 seconds to fill back in. This process should take about 10 minutes.
Remove from the heat and transfer to a bowl, cover with plastic wrap, and let it cool. Transfer to the fridge once it is no longer hot.
Once the mixture is completely chilled, use a teaspoon to scoop some of it out, about the size of a walnut. Roll into a ball in your hands.
Roll the chocolate ball in vermicelli, sprinkles, chopped nuts, or a topping of your choosing. Place into a paper liner and enjoy.
Notes
It's important not to use too high a heat to cook the brigadeiro mixture, or it could burn. Make sure you are stirring constantly while cooking, and try to scrape the bottom of the pan while you go. I like to use a wooden spatula for this to cover more surface area, but any heatproof utensil will do. The mixture is easiest to roll when chilled, but if it is too firm, let it sit at room temperature until it is pliable enough to roll into balls.