To prepare syrup, combine all ingredients in a small, heavy saucepan, whisking until smooth. Heat over medium heat, until sugar dissolves and syrup is smooth.
To prepare cake, pour stout over currants; cover and soak until plump.
Drain currants, reserving stout. Add stout to a small saucepan. Whisk in 1/3 cup cocoa and bring to a simmer. Remove from heat; add semisweet chocolate, stirring until chocolate melts. Cool slightly. Stir in buttermilk.
Preheat oven to 350°F
Combine 2 tablespoons cocoa, 2 tablespoons sugar and 2 tablespoons flour. Coat two 8- or 9-inch square or round cake pans with cooking spray; dust with cocoa mixture.
Beat butter with a mixer at medium speed until smooth. Gradually beat in 1/4 cups sugar until well blended. Beat in eggs one at a time. Beat in vanilla.
Combine 2 cups flour with baking soda, baking powder and salt. Add flour mixture to butter mixture alternately with chocolate mixture, stirring until blended. (Batter may look curdled.) Stir in currants.
Divide batter between pans. Bake 25 to 30 minutes, until a wooden pick inserted in the center comes out clean. Cool in pans on a wire rack 10 minutes; invert onto rack.
Poke tops of cake layers with a skewer or toothpick. Spoon Drizzling Syrup over tops. Place one layer on a platter. Spread warmed jelly over layer on the platter. Chill 30 minutes. Cover jelly with about a quarter of the Bittersweet Icing. Place second layer on top.
Ice top and sides of the cake with the remaining icing. Press nuts into sides of cake.