Directions

1. Preheat oven to 300 degrees. Coat a 12-by-17-inch rimmed baking sheet with cooking spray. Line with parchment paper. Spray and flour parchment, tapping off excess; set aside. In a food processor, pulse walnuts and flour until coarsely ground; set aside.
2. In the bowl of an electric mixer, whisk egg yolks with 5 tablespoons sugar until thick and pale. Beat in vanilla. In a clean mixing bowl fitted with a clean whisk, beat egg whites with salt until soft peaks form. Gradually add remaining 5 tablespoons sugar, beating until stiff glossy peaks form. Fold egg whites into yolk mixture in three equal batches, and add walnut mixture with last batch.
3. Spread batter evenly on prepared baking sheet. Bake until top is golden and springs back when touched, about 30 minutes. Run a small sharp knife around edges of cake; invert cake onto a clean kitchen towel dusted with confectioners’ sugar. Peel off parchment paper. Starting at a short side, gently roll the cake into a log, incorporating towel. Transfer to a wire rack, and let cool completely, about 1 hour.
4. Reserve 1 1/2 cups mascarpone filling. Unroll cake and spread remaining filling on top, leaving a 1/2-inch border on all sides. Carefully reroll cake (without towel). Arrange, seam-side down, on a parchment-lined baking sheet. Using a serrated knife, trim end of log. Spread reserved filling over log. Working quickly, arrange chocolate bark all over log, overlapping pieces slightly. Refrigerate until firm, at least 1 hour. Garnish with acorns and leaves, if desired; dust with confectioners’ sugar before serving.