In a microwave, melt candy bar with chocolate syrup; stir until smooth. Set aside to cool.
In a large bowl, cream butter and sugar until light and fluffy. Add eggs, one at a time, beating well after each addition. Beat in chocolate mixture and vanilla. Combine the flour, baking powder, baking soda and salt; add to creamed mixture alternately with buttermilk, beating well after each addition.
Pour into a greased and floured 10-in. fluted tube pan. Bake at 350° for 50-60 minutes or until a toothpick inserted in the center comes out clean. Cool in pan on a wire rack for 15 minutes. Remove from pan and cool completely. Dust with confectioners' sugar if desired.