Special equipment: An offset spatula and 2 1/2-inch wide acetate ribbon, see Cook's Note
To make collars: Cut 2 1/2-inch wide acetate ribbon into 6 (10-inch) lengths. Lay a piece of acetate on a flat surface and spread 1 ounce of melted chocolate evenly over the entire surface using an offset spatula. Form the acetate into a teardrop shape, chocolate side in. Pinch the 2 ends of the acetate together. Sit the chocolate collar on a parchment or waxed paper lined tray. Repeat with the rest of the acetate. Refrigerate the collars until chocolate becomes firm.
To make chocolate mousse: Combine the water and sugar in a pot and bring to a boil, stirring to dissolve the sugar. Simmer for 5 minutes to create a simple syrup. Cool slightly. Whisk the simple syrup into the egg yolks until light and foamy. Fold in the espresso, Grand Marnier, and melted chocolate.
Beat the egg whites until they hold a soft peak, then fold into the chocolate mixture.
Whip the cream until it holds a soft peak, and then fold into the mousse. Spoon the mousse into the collars. Cover and refrigerate for at least 3 hours. Using an offset spatula gently transfer the mousse to a plate. Carefully remove the acetate. Serve immediately.
Acetate ribbon is a medium gauge clear plastic often used for cake decorating and chocolate work. Sold in rolls, it can be found in baking supply stores.