Make cake: Mix chopped dried figs, dried currants and brandy in medium bowl. Let stand 1 hour.
Preheat oven to 350°F. Butter 15 1/2 x 10 1/2 x 1-inch jelly roll pan. Line with parchment. Butter and flour parchment. Drain fruit mixture; reserve soaking liquid. Mix sugar, flour, cocoa, baking soda, baking powder, and spices in large bowl. Whisk prune juice, buttermilk, butter and eggs in medium bowl to blend. Add prune juice mixture to dry ingredients and whisk to blend. fold in drained fruit.
Pour batter into prepared pan; smooth top. Bake until tester inserted into center o cake comes out clean, about 25 minutes. Cool in pan on rack.
Make frosting: Bring reserved fruit soaking liquid, cream and butter to simmer in heavy large saucepan, whisking until butter melts. Remove from heat. Add chocolate and whisk until melted and smooth.Add sour cream and whisk to blend. Refrigerate frosting until thick enough to spread, stirring occasionally, about 30 minutes.
Run sharp knife around pan sides to loosen cake. Turn out cake onto work surface. Peel off parchment. Using serrated knife, cut cake crosswise into three 5 10-inch rectangles. Place 1 cake piece on platter. Tuck waxed paper strips under cake on all sides to protect platter. Spread 1/2 cup frosting over cake. Sprinkle with 1/4 cup chopped walnuts. Top with second cake piece. Spread 1/2 cup frosting over. Sprinkle with 1/4 cup chopped walnuts. Top with third cake piece. Spoon 3/4 cup frosting into pastry bag fitted with small star tip and reserve. Spread remaining frosting over top and sides of cake. Press remaining chopped walnuts onto sides of cake. Remove waxed paper strips. Using reserved frosting, pipe 5 rosettes atop cake along each ling edge, spacing evenly. Place walnut halves between rosettes. Refrigerate 1 hour to set frosting. (Can be prepared 1 day ahead. Cover and keep refrigerated. Let stand 2 hours at room temperature before serving.)