Step 1Heat the oven to 350 degrees. Butter 2 (8-inch) square pans and dust with cocoa powder.
Step 2Melt the chocolate in a double boiler set over, but not touching, simmering water. Cool.
Step 3Cream together the butter and sugar in the bowl of an electric mixer until light and fluffy. Add the eggs, 1 at time, beating well after each addition. Blend in the vanilla.
Step 4Stir together the cake flour and salt. Combine the cider vinegar and milk and let stand 1 minute. Blend the flour into the butter mixture alternately with the soured milk, mixing well. Add the melted chocolate, blending thoroughly.
Step 5Dissolve the baking soda in the malt vinegar in a bowl large enough to accommodate foaming of the mixture. Add to the batter, mixing well. Divide the batter equally between the pans.
Step 6Bake until a toothpick inserted in the center of the cakes comes out clean, 30 minutes. Cool the cakes in the pans 10 minutes, then remove and cool completely on wire racks.
Step 1Melt the chocolate in the top of a double boiler set over, but not touching, simmering water. Cool.
Step 2Combine the butter and powdered sugar in a food processor. While processing, add the milk, vanilla and melted chocolate through the feed tube. Blend until smooth and creamy. (Makes about 2 1/3 cups.)
Step 1Place 1 cake layer, top-side down, on a serving platter. Spoon about 1/3 cup of the frosting onto the layer and spread evenly to the edge. Sprinkle with chopped walnuts. Place the other layer, top-side up, on the frosted layer. Frost the sides of the cake, then top, covering completely.