Spray a 10-inch springform pan with cooking spray.

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 half of the nuts, reserving 1/2 for the top of the Turtle brownie ice cake. Scoop all of the ice cream on top of the hot fudge and nuts, spreading evenly with a knife or spatula. Freeze until ready to eat.
Remove from freezer and remove the sides from the pan, using the bottom of pan as a serving plate.
The remaining hot fudge can then be heated in the microwave and dribbled on top of the cake and sprinkle with the remaining nuts before serving.