In a large heavy saucepan whisk together the brown sugar, cornstarch, and salt. Whisk in the milk, cream and eggs until fully blended. Bring to a boil over medium-high heat, whisking very frequently, then continue to boil, whisking constantly, for 1 minute.
Remove from the heat and whisk in the vanilla. Pour into one large bowl or 6 individual serving dishes, then cover surface with plastic wrap and refrigerate until chilled, at least 1 hour for the smaller dishes, 2 1/2 hours for a bigger bowl.
Notes
For this I used a fine microplane, which really just results in a dusting of dark chocolate, which is all that was needed in terms of flavor and looks. For a bit more crunch and a different kind of flavor, come cocoa nibs would be great.