With an electric mixer, cream the butter and sugar together until pale and fluffy. Add the eggs one at a time, beating well after each addition. Stir in the vanilla. Combine the cornmeal, flour, baking soda and salt, mixing well. Add the flour mixture to the butter mixture in thirds, alternating with the buttermilk, mixing well after each addition. Stir in the chopped nuts and mix well. Spread the batter evenly in a greased 13-inch by 9-inch baking pan. Bake in a pre-heated 350-degree oven for 30 minutes until firm and lightly browned. Remove from the oven and pour the maple syrup evenly over the top of the cake. Return to the oven and bake another 5 minutes. Remove from oven and let cool completely before serving.