Preheat the oven to 350°. Grease an 8-by-3-inch springform pan. Dust with flour; tap out any excess. In a small heavy saucepan, melt the chocolate with the safflower oil over low heat until just warm. Stir to blend and let cool to room temperature. In a food processor, combine the walnuts and 2 tablespoons of the sugar. Process briefly until just ground, 10 to 12 seconds. In a large bowl, combine the ground nuts cinnamon and bread crumbs; toss well to combine. In another bowl, beat the egg yolks lightly. Gradually whisk in 1/2 cup of the sugar, beating until the mixture is pale and thick, about 5 minutes. In a large bowl, beat the egg whites until soft peaks form. Gradually add the remaining 2 tablespoons sugar and continue to beat until stiff, glossy peaks form. Fold the cooled chocolate mixture and the Cognac into the beaten egg yolks. Pour over the bread crumb mixture and fold together until blended. Stir one third of the beaten whites into the mixture to lighten it. Fold in all of the remaining egg whites until no white streaks remain. Quickly scrape the batter into the prepared cake pan and bake in the middle of the oven for 45 to 50 minutes, or until the surface is covered with small cracks and a cake tester inserted in the center comes out clean. Transfer the cake to a wire rack and let cool for 10 minutes. Remove the sides of the pan and let cool completely. (The cake can be made to this point , up to a day ahead. Wrap well.) Invert the cake and carefully remove the springform bottom. With a long serrated knife, split the cake in half horizontally into 2 equal layers. Spread the Mocha-Walnut Buttercream over one cut cake layer and sandwich the cake together again. Place the cake, flat-side up, on a rack over a jelly-roll pan. Pour the warm Chocolate Glaze over the cake. Using a metal spatula, spread it evenly over the top and sides. Refrigerate until the glaze sets, about 30 minutes. Using a metal spatula (your fingers may melt the chocolate), arrange the leaves decoratively on top of the cake. NOTE: To toast walnuts, place them on a baking sheet and bake in a 350° oven until fragrant, about 10 minutes.