Crush 12 graham crackers and 12 soda crackers. Mix with 1/2 stick melted butter and press in 9 x 13 inch pan. Whip 2 (3 1/2 ounce) packages instant vanilla pudding with 2 cups milk for 1 minute. Stir in 1 quart butter brickle ice cream into pudding until well mixed. Pour into pan. Set in refrigerator about 1 hour. Spread 8 ounces Cool Whip on set topping. Crush 2 Heath bars and sprinkle over Cool Whip. Add walnuts, if desired. Keep refrigerated until ready to serve.