Line a 9x9 pan with plastic wrap, leaving plenty of extra draping over the sides. Take the ice cream out of the freezer and let it soften for 10-15 minutes. Don't let it get too soft or when it refreezes, it will get icy.
Line the pan with a thin layer of ice cream, then graham crackers.
Spread 1/3 of the hot fudge across the graham crackers and put the pan in the freezer for 15 minutes. Repeat layers, ending with ice cream on top. Freeze for at least 4 hours.
To serve, dip pan in hot water to loosen edges. Invert ice cream cake onto a serving plate, remove plastic wrap and frost with whipped topping. Dust with cocoa powder or drizzle with hot fudge, if desired.