- 2 cups walnuts, lightly toasted and cooled
- 2 tablespoons walnut oil or olive oil pinch kosher salt
- 5 ounces 64–70% bittersweet chocolate, melted
- 3 egg yolks
- ⅓ cup granulated sugar
- ¼ cup water
- 1½ cups heavy cream, whipped to medium peaks
- 6 ounces 64–70% bittersweet chocolate
- ⅓ cup water
- 1 teaspoon vanilla extract
- 6 egg whites
- ¼ teaspoon cream of tartar
- ¾ cup granulated sugar
- 6 egg yolks
- cocoa powder, as needed
Place nuts, oil and salt into a food processor; pulse until it forms a paste. Add melted chocolate, process until combined, then place into a large bowl and set aside. In a mixer bowl, whip yolks on low speed. In a small pot, combine sugar and water, place over medium heat and bring to a boil. Boil for 1 minute; then slowly pour into whipping yolks. Whip on high speed until cooled. Fold yolks into walnut paste. Fold in cream and set aside. Use right away or refrigerate, but do not chill more than 1 hour or it will be hard to spread.
Preheat oven to 375°. Line an 11- by 17-inch cookie sheet with parchment paper.
Place chocolate and water in a medium-sized bowl and melt over a water bath. Whisk lightly, add vanilla, and leave over the water bath. Place egg whites in a mixing bowl and whip until foamy. Add cream of tartar, beat to soft peaks; add sugar and beat until stiff.
Make sure chocolate has not cooled off, or it will cause lumps. Whisk yolks into melted chocolate. Fold ¼ of whites into chocolate completely. Add remaining whites and fold gently, until just combined. Spread onto prepared pan and bake 8–10 minutes, until it springs back when touched. Remove from oven and cool completely in pan on a wire rack.
Loosen edges of cake with a knife or offset spatula. Using a sifter, finely dust with cocoa powder, cover with foil and flip onto the back of another pan or cutting board. Using an offset spatula, spread mousse evenly over cake. With the long edge nearest you and starting with the edge farthest away from you, roll the cake towards you, creating a log. Cinch the foil around the log to keep it tight while it is setting up. Refrigerate and chill for at least 3 hours. Can be kept, refrigerated, for 3 days.