In a large bowl blend brown sugar, flour, and butter with a pastry blender. Remove 1 cup and set aside for topping.
Mix baking soda into buttermilk. Add egg, vanilla, and salt, stirring to blend. Mix with original dry ingredients in bowl.
Pour into 9" x 11" greased baking pan. Top with reseved cup of the flour-sugar mixture. Finely crush the Heath bars (still frozen) sprinkle over the top of the cake. Add nuts if desired.
Bake at 350�f for 35 minutes. Serves 8 to 10.