Preheat oven to 350 degrees F. In a large bowl, using a pastry blender, mix the flour, granulated sugar, brown sugar and butter together until fine particles form. Remove 1/2 cup of the mixture to use to top cake. Blend in the buttermilk, egg, vanilla, baking soda and baking powder and transfer the batter to a greased and floured 9x13 inch pan. To make the topping: Combine the crushed candy bars, nuts and reserved flour/sugar mixture. Sprinkle over the batter and bake for 28-30 minutes, or until the cake springs back when touched in the center. Remove from the oven and cool completely. Makes 15 servings.