Melt chocolate bars in syrup over hot, not boiling, water. Cream butter; gradually beat in sugar until light and fluffy. Add eggs, one at a time, blending well after each addition. Mix baking soda with buttermilk. Add to creamed mixture alternately with flour. Blend in chocolate mixture and vanilla. Pour into greased and floured 10-inch tube pan. Bake in 350° oven for 1 hour and 20 minutes. Cool in pan 10 minutes. Turn onto rack to finish cooling. Sprinkle with confectioners' sugar.