Place sweetened condensed milk, butter and cocoa powder in a saucepan. Turn on flame to medium/low, stir to combine. Keep stirring for about 15 minutes total. The mixture become thick. Transfer mixture to a bowl and allow to cool.
Grease your hands and form into walnut sized balls. Roll in sprinkles, coconut, nuts or any other topping of your choosing. I'm a fan of coconut taste-wise, but sprinkles are cute.
Makes about 18 brigadeiros. Serve immediately or store in an airtight container for up to 1 week.