Measure out roughly 2 tbsp of butter and melt in the pan. Set the stove top to medium-low. Once the butter is melted, add the whole can of condensed milk. Put the cocoa powder in a small bowl or mug and add some water to make into a paste, then add to the pan of condensed milk.
Stir with the wooden spoon — you will need to stir for at least 20 minutes and it can take as long as 40 minutes! Stir in a circular motion and make sure you scrape away the mixture from the sides of the pan so it does not burn. The brigadeiro will get thicker and reduce in volume.
You’ll know it’s ready when a) you divide it in half with the spoon and it stays parted for a few seconds and b) you scoop up some of the fudge with the wooden spoon, turn the spoon upside down, and it stays stuck for an instant before falling back into the pan. When you’re sure it’s ready, spread the contents of the pan on a plate and put it in the fridge for about 30 minutes; you want it to be firm.
To make the balls, spread some butter on your hands and make sure to cover your fingers. Using a small spoon, scoop up some of the fudge and roll it into a little ball with your hands. Now roll the ball in the chocolate sprinkles to cover it completely. Place your brigadeiro in a paper cup. Repeat the process – it should yield about 25 brigadeiros. Put them in the fridge for about 15-30 minutes to make them firmer, then all that’s left to do is enjoy.