To serve the mousse in 5- to 6-ounce ramekins, cut 6 strips of parchment or waxed paper about 4 inches wide and 1 inch longer than the circumference of the ramekins. Fold the strips lengthwise in half Cut 6 long pieces of twine. Wrap each ramekin in a strip of paper, allowing the paper to extend about 1 inch over the top of the ramekin, and tie the collar securely with a piece of twine.
Place ¾ cup of the cream in a small saucepan and bring to a simmer. Turn off the heat, add the chocolate, and let stand for 2 minutes. Stir until the chocolate is melted.
In a medium bowl, whisk the egg yolks and sugar until thickened.
In a saucepan, bring the milk to a simmer over medium-low heat. As soon as it begins to simmer, add a few tablespoons of the warm milk to the yolk-sugar mixture, whisking constantly. Pour about half the warm milk into the bowl, while whisking, then return the mixture to the saucepan and stir constantly for about 8 to 10 minutes, or until the custard thickens and reaches 175°F.
Meanwhile, fill a large bowl with ice water. Strain the custard through a fine-mesh strainer into a large bowl set over the ice water. Stir until it is cool to the touch, then remove from the ice water.
In a large bowl, whip the remaining 1¾ cups cream until it forms very soft peaks. Stir the chocolate mixture if it has separated, and fold into the cooled egg mixture. Fold in the whipped cream.
Spoon the mousse into the prepared ramekins or into a serving bowl. If using the ramekins the mousse should extend about ½ inch over the rim of each ramekin. Place the ramekins on a tray and lay a piece of aluminum foil over the top. Freeze for at least 6 hours or up to 2 days.
About 5 to 10 minutes before serving, remove the mousse from the freezer. Remove the collars, and press the nibs or ground chocolate around the sides of the mousse.