Spread some butter on an 8 or 10-inch plate and set aside.
Put 2 tablespoons butter in a medium pot and bring heat to medium. Once melted add the sweetened condensed milk. Then add the cocoa powder to a small mesh sifter and sift it into the pot. Stir to incorporate, then add the cinnamon. Reduce heat to low and cook for 10 minutes stirring frequently. It should get very thick.
Pour the batter onto the greased plate and put it into the fridge for 1 hour.
Take the plate out of the fridge. Roll the fudgy mixture into balls, then roll the balls in the chocolate sprinkles. Repeat until all the batter is used up. The brigadeiro is ready to serve or store in the fridge.