Cook over medium-low heat, stir vigorously the sweetened condensed milk, margarine and cocoa powder.
Cook the mixture until it thickens enough to show the pan bottom during stirring.
Pour the mixture in a greased dish and let it cool to room temperature. Take small amounts of the mixture with a teaspoon and make 1 ½ inch balls.
Roll the balls over chocolate jimmies to decorate.
Hint: Grease your hands with margarine to make the balls easily.
If the balls don't hold the shape and flat down like coins, it means that you did not cook enough. Cook additional five minutes, take a small sample and dip it in a glass with cold water. If you cooked enough, this sample should hold the shape after cooling down in the water.
The cheapest , low grade sweetened condensed milk is the best to make the brigadeiro because they have more starch on them, which helps the candy to hold the shape.
In US, buy the sweet condensed milk from the Walmart brand "Great Value", cook this brand for 20 minutes in medium heat stirring continuously and your brigadeiro is with the right consistency.