Whisk the condensed milk and cocoa powder together until homogeneous (no lumps). Then, stir in the butter.
Stove: Cook in a medium non-stick saucepan over medium heat (large burner), stirring constantly, until the mixture thickens and starts to peel away/show the bottom of the pan when you scrape it with your wood spoon (about 5-8 minutes).
Microwave: Pour mixture into a deep microwaveable bowl. If 900 watts, let cook on full power for about 6 minutes -- removing and stirring at least every 2 minutes, or until thick enough to be rolled (when it cools down, mixture will be thicker).
While it is cooking, do not leave the microwave unattended because mixture will rise and possibly bubble over, making a mess. Every time it rises, pause until mixture settles back down. If 1100 watts, cook at 80 % power. Double batches take more time to cook
When brigadeiro is ready, remove either from the stove top or microwave, mix in the vanilla extract and spread mixture onto a greased plate.
Let cool to room temp before starting to roll them into balls with greased hands. Use a 1/2 ( or 1 ) tbsp as measurement.
Then, dredge gently in the chocolate sprinkles until totally covered, and place into paper bonbon cups.