1. Process walnuts and oil in food processor with blade and pulse, scraping as needed, until it forms a paste. Add chocolate and process until mixed.
2. Whisk yolks lightly by hand in the bowl of a heavy duty mixer. Boil a couple inches water in a pot which can hold the yolks bowl as a double boiler. Keep water warm in preparation for next step.
3. Boil water and sugar together. Remove from heat, very gradually add to eggs, whisking yolks all the while. Put bowl in simmering double boiler pot, continue whisking until mixture is foamy and hot to the touch.
4. Remove eggs from heat, mix with mixer until eggs are pale, holds a soft ribbon, and is cool to the touch.
5. Fold in walnut-chocolate mixture.
6. Whip cream until soft peak stage. Fold delicately but thoroughly into walnut mixture. Refridgerate until needed.

From Baking with Julia, a really fancy yummy cookbook. This mousse is merely the filling for a Vanilla Chiffon Roll (6 servings for the roll).