Combine buttermilk and soda in a small mixing bowl; stir well, and set aside.
Combine eggs and persimmon pulp in a large mixing bowl; stir well. Sift together flour, baking powder, cinnamon, and salt in a medium mixing bowl. Add dry ingredients to egg mixture alternately with buttermilk mixture, beginning and ending with flour mixture. Stir well after each addition. Stir whipping cream, sugar, butter, vanilla, and walnuts into batter; beat well.
Pour batter into a well - greased 2 1/2-quart shallow baking dish . Bake at 350° for 45 minutes or until a knife inserted in center comes out clean. Cool in dish 10 minutes. Cut into 2 1/2-inch squares, and transfer to a serving plate. Serve with Creamy Vanilla Sauce.