In a saucepan over low heat, stir chocolate syrup and candy bar until candy bar is melted; cool. In a large bowl, cream butter and sugar. Add eggs, one at a time, beating well after each addition. Add chocolate mixture and vanilla; mix well. Combine flour and baking soda; add to creamed mixture alternately with buttermilk. Fold in pecans.
Pour into a greased and floured 10-in. fluted tube pan. Bake at 350° for 70-75 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing from pan to a wire rack to cool completely.
For frosting, in a saucepan, melt butter. Stir in cocoa until smooth. Add milk. Bring to a boil. Remove from the heat. Stir in the confectioners' sugar and vanilla until smooth. Spread over cake. Sprinkle with pecans.