In a large mixing bowl, combine Oreo cookies and melted butter until well blended. Press firmly into a greased 13x9-inch dish.

Allow vanilla ice cream to soften, then spread it evenly over the crust. Freeze for 2-3 hours until firm.

Warm the hot fudge topping in the microwave and drizzle over the ice cream layer. Freeze for 1-2 hours until set.

Spread whipped topping over the fudge layer, sprinkle reserved cookie crumbs on top, and freeze for at least 2 hours.

Let the cake sit at room temperature for about 10 minutes before slicing into squares to serve.