For Cake: Preheat oven to 325F. Butter 9-inch-diameter springform pan with 2 1/2-inch-high sides. Line bottom with parchment. Butter parchment. Dust pan with flour. Melt butter and chocolate in heavy large saucepan over low heat, stirring constantly until smooth and melted. Remove from heat and whisk in sugar, eggs and ground nuts. Pour batter into prepared pan. Bake cake until toothpick inserted into center comes out with very moist crumbs attached, about 45 minutes. Cool completely on rack (cake will fall and crack). Release cake pan sides. Turn out cake onto work surface. Peel off parchment. Cut off outer 1/2-inch edge of cake. Clean cake pan. Reattach cake pan sides; brush sides of pan with vegetable oil. Set cake in center of pan (there will be space around cake that will be filled in with mousse). Refrigerate cake. For Mousse: Boil tangerine juice and sugar in heavy large saucepan until reduced to 1 1/2 cups, about 20 minutes. Meanwhile, sprinkle gelatin over lemon juice in small bowl. Let stand 10 minutes to soften. Add gelatin mixture to tangerine juice mixture and stir to dissolve. Add 14 ounces white chocolate and whisk over low heat just until melted. Whisk in sour cream. Pour into large bowl. Chill mixture until cool and thickened but not set, whisking occasionally, about 1 hour 45 minutes. Beat chilled whipping cream to stiff peaks. Fold into white chocolate mixture. Pour mousse over cake in pan, covering top and sides completely. Refrigerate cake overnight. Using vegetable peeler, shave 9 ounces white chocolate into thin curls (if white chocolate is too hard to shave, place briefly in warm place to soften slightly). Run small sharp knife around pan sides to loosen cake. Release pan sides. Place cake on platter. Sprinkle chocolate curls over top of cake. Press more chocolate curls around sides of cake, covering completely. Place cocoa powder in fine sieve. Dust cocoa very lightly over chocolate curls. (Can be prepared 1 day ahead. Cover and refrigerate.) Garnish cake with raspberries and mint sprigs. Serve cold. Serves 12. Bon Appetit December 1992