Prepare the syrup: Combine Guinness stout, brown sugar, cocoa powder, and vanilla in a saucepan.

Whisk until smooth and heat over medium until sugar dissolves.

Pour stout over currants, cover, and soak until plump.

Drain currants, reserving the stout.

Add the stout to a saucepan.

Whisk in 1/3 cup cocoa powder and bring to a simmer.

Remove from heat; add semisweet chocolate and stir until melted.

Cool slightly and stir in buttermilk.

Preheat oven to 350F.

Combine 2 tablespoons cocoa, 2 tablespoons sugar, and 2 tablespoons flour.

Grease and dust two 8- or 9-inch cake pans with the cocoa mixture.

Beat softened butter with a mixer until smooth.

Gradually beat in sugar until well blended.

Beat in eggs one at a time and add vanilla.

Combine flour, baking soda, baking powder, and salt.

Add flour mixture to butter mixture alternately with chocolate mixture, stirring until blended.

Stir in currants.

Divide batter between pans and bake for 25-30 minutes, until a toothpick comes out clean.

Cool in pans for 10 minutes; invert onto rack.

Poke tops of cake layers with a skewer.

Spoon syrup over tops.

Place one layer on a platter and spread warmed red currant jelly over it.

Chill for 30 minutes.

Cover jelly with bittersweet icing.

Place the second layer on top.

Ice top and sides of the cake with the remaining icing.

Press chopped walnuts into the sides of the cake.