Chocolate Brigadeiros

1 In a medium heavy-bottomed saucepan, place the condensed milk, butter, heavy cream, and corn syrup and bring to a boil over medium heat.
2 When the mixture starts to bubble, add the chocolate and the cocoa powder. Whisk well, making sure there are no pockets of cocoa powder. Reduce the heat to low and cook, whisking constantly, until it is the consistency of a dense fudgy batter, 8 to 10 minutes. You want the mixture to bubble toward the end, so it's important to use low heat or the sides of the pan will burn the fudge. If you undercook it, the brigadeiro will be too soft; if you overcook it, it will be too chewy. It is done when you swirl the pan and the mixture slides as one soft piece, leaving a thick burnt residue on the bottom.
3 Slide the mixture into a bowl (without scraping the bottom) and let cool at room temperature. Cover the bowl with plastic wrap and chill in the refrigerator for at least 4 hours.
4 Scoop the mixture by the teaspoonful and, using your hands, roll each into a little ball about 3/4 inch in diameter (about the size of a chocolate truffle).
5 Place the sprinkles in a bowl. Roll 4 to 6 brigadeiros at a time through the sprinkles, making sure they cover the entire surface. Store in an airtight plastic container for 2 days or up to 1 month in the refrigerator.