Sprinkle the yeast into the water and stir until dissolved. Beat in the milk and 1/2 cups flour until smooth. Cover and let rise in a warm place until light and spongy, about 30 minutes.
Meanwhile, cream together the butter and sugar. Add 1 egg at a time, beating well after each addition. Add yeast mixture, remaining flour, melted chocolate, baking soda, salt and vanilla. Beat until well-blended. Turn into a well-greased 10-inch tube pan. Cover and let rise in a warm place, free from draft, until light and bubbly, about 1 hour. Bake in a preheated 350° F oven for 50 minutes, or until a cake tester inserted in center comes out clean. Cool on a cake rack 15 minutes then turn out onto the rack. Let stand until cold.
If after this, you'll want to make something more typical like cookies!