Combine the flour, the 1/3 cup brown sugar, the baking soda, baking powder, nutmeg, cloves, and salt. Set aside.
Combine dates and margarine. Add the 1 cup boiling water. Stir until margarine melts. Stir in vanilla. Stir date mixture and black walnuts into the flour mixture just until smooth. Spread in an ungreased 2-quart square baking dish.
Stir together the 1-1/2 cups boiling water and the 1 cup brown sugar. Pour evenly over batter in pan. Bake in a 350 degree F oven about 45 minutes or until cake tests done. Serve cake warm. If desired, serve with whipped cream. Makes 9 servings.