Preheat oven to 350~F. Cream butter and gradually add the light brown sugar and sugar. Blend in flour and 1/4 ts salt; mix well. Reserve 1 cup of this mixture for topping. Combine buttermilk, baking soda, 1/8 ts salt, egg and vanilla. Stir into remaining butter-sugar mix. Fold in 2 finely chopped Heath bars. Turn into a buttered 13 x 9 pan and sprinkle with the 1 cup reserved topping, then with the pecans. Top with additional 6 crushed Heath bars. Bake for 35 - 40 minutes. Serve with cocoa-flavored whipped cream or with coffee ice cream.