Mix turtle brownie mix according to directions on box. Pour into prepared pan and bake at recommended temperature on box. Remove from oven 5 to 10 minutes before the recommended time on the box so that the brownie is still gooey. Let cool completely.
Remove ice cream from freezer and let it soften about 5 minutes. While it is softening, spread 1/2 of the hot fudge topping on top of the brownie and sprinkle with some of the nut topping. Scoop all of the ice cream on top of the brownie, spreading evenly with a knife or spatula. Freeze in freezer until ready to eat. The remaining hot fudge can then be heated in the microwave and dribbled on top of the cake before serving.