In bowl of electric mixer, cream butter and sugars. Add eggs one at a time, beating well after each. Stir dry ingredients together. Combine starter and vanilla; add alternately with dry ingredients to sugar mixture. Spread in greased 11 1/2 x 7 1/2 x 1 1/2" pan. Sprinkle chocolate and nuts on top. Bake in preheated 350F oven 30 min.