make the mixture
Place a pan over medium heat. Add the butter and cook until the butter melts. Add the condensed milk and stir to combine. Sift the cacao into a small bowl. Reduce the heat and add the cacao. Cook for about 6 - 8 minutes while regularly stirring for the mixture to thicken. The mixture is ready when you can see the bottom of the pan for 2-3 seconds when dragging a spatula through the bottom.
cool the brigadeiro mixture
Transfer the mixture to a large bowl and place in the fridge for at least 2 hours (or overnight).
coat with sprinkles
Add chocolate sprinkles to a deep plate. Grease your hand with butter. Scoop a teaspoon of the mixture and shape it into a small ball. Repeat the process until you use all the mixture. Coat the balls in chocolate sprinkles and arrange them on a serving plate. Keep refrigerated until serving. Ten minutes before serving, take the dessert from the fridge.