Oil an 8-inch pie dish or a plate with butter and set aside.

In a small bowl, mix cocoa powder with water to form a paste.

In a nonstick heavy pan, melt the butter.

Add the sweetened condensed milk and the cocoa mixture to the pan.

Stir constantly with a wooden spoon until the mixture boils.

Reduce the heat to medium-low and cook for 10 to 15 minutes, stirring constantly.

Continue stirring until the mixture is thick, shiny, and starts to pull away from the bottom and side of the saucepan.

Check for doneness by scooping some mixture with a spoon and turning it upside down - it should hold briefly before falling.

Pour the mixture into the oiled pie dish.

Refrigerate until firm, about 30 minutes.

Place chocolate sprinkles in a small bowl.

Lightly butter your hands.

Roll about a teaspoon of the mixture between your palms to form 1-1/2-inch balls.

Roll the balls in the chocolate sprinkles to cover them completely.

Place each Brigadeiro in a mini paper baking cup.

Store tightly covered at room temperature for up to 2 days or in the refrigerator for up to 1 week. Can also be frozen for up to 30 days.