A mousse from The New Best Recipe helped produce the dense velvety mousse I was looking for. I used two large eggs, 3 ounces of bitter chocolate, 1 ounce of unsalted butter, 3/4 cup of cream and 1 tblsp sugar.
The key difference from my usual recipe is the use of butter, although some other changes may have helped stiffen the mousse. Usually I heat up a yolk, sugar and milk mixture to 80C before mixing in the chocolate. In this recipe I melted the chocolate first and beat in the uncooked yolks one at a time. Instead the sugar was beaten into the stiff egg whites which where warmed slightly before beating. The cream, on the other hand, was chilled and beaten in a metal bowl set in iced water.
Following the recipe I beat a portion of the egg white into the chocolate/yolk mixture before folding in the rest. I took Robert's advice and folded in the cream in a portion at a time. I did this with both the egg whites and the cream.
Presentationally my mousse was not close to the picture posted in my question, but from the shape the mousse kept while I scooped out spoonfuls, I can say that the texture was comparable. On a whim I used three times as much cream as the recipe required. A richer mousse would certainly have been better so I hope that the specified 1/4 cup of cream can be used without radically altering the texture.