Set a small saucepan filled with 2 cups of water over a medium low heat. Place the egg yolks and hazelnut liqueur into a medium-sized mixing bowl, and beat with a whisk. Place the bowl over the saucepan and continue to beat the eggs until they start to thicken. The egg should take on the consistency of a hollandaise sauce but since it is a sweet sauce, it will be called a sabayon. As you draw the whisk up, the sabayon should fall off the whisk in ribbons, and when it lands on itself in the bowl, it should hold its shape for a moment.

Once you've completed the sabayon, fold the warm melted chocolate and salt into the bowl and allow it to cool. Be sure to not over work the sabayon as you fold the chocolate into it or it will deflate and you will lose air volume.

In a separate clean bowl, whisk together the heavy cream, sugar and vanilla extract to make whipped cream. Beat the cream until stiff peaks form and then fold the whipped cream into the sabayon and chocolate mixture.

Line a 9 by 5-inch loaf pan with plastic wrap, being sure that you have about 3 inches of overhang on all sides of the pan. Pour the mousse mixture into the prepared loaf pan, raise the loaf pan about 3 to 4 inches off the workstation and drop. Do this 2 or 3 times to force out any air bubbles that may have formed in the pan. Drape the overhanging plastic over the top of the mousse and overlap it with the plastic wrap on the other side. Place the terrine in the freezer and freeze for at least 4 hours, and up to 2 days.

To serve the terrine, remove from the freezer, peel back the plastic wrap and invert the loaf pan onto a clean cutting board. Peel back the remaining plastic wrap and slice the terrine using a hot, long thin bladed knife. Make the slices about 3/4-inch thick, and place on a clean plate, garnished with the pistachios, whipped cream and mint leaves.