In a heavy-bottomed saucepan, combine the sweetened condensed milk, unsweetened cocoa powder, and butter. If using, add the pinch of salt.

Stir the ingredients together until the cocoa powder is fully incorporated and there are no lumps. It's best to do this off the heat.

Place the saucepan over medium-low heat. Cook, stirring constantly with a silicone spatula, scraping the bottom and sides of the pan to prevent sticking and burning.

Continue cooking and stirring for about 10-15 minutes, or until the mixture thickens significantly. You'll know it's ready when you can drag your spatula through the mixture and the path stays open for a few seconds before closing, or when it starts to pull away from the sides and bottom of the pan.

Pour the cooked brigadeiro mixture onto a buttered plate or a piece of parchment paper. Spread it out slightly to help it cool faster.

Let the mixture cool completely. This can take about 1-2 hours at room temperature, or you can speed it up by refrigerating it for about 30-60 minutes.

Once cooled, lightly grease your hands with butter. Scoop about a tablespoon of the mixture and roll it into a small ball between your palms.

Roll each ball in chocolate sprinkles until evenly coated.

Place the finished brigadeiros in small paper candy cups or on a serving platter.