Grease an 8x8-inch or 9x9-inch baking dish with non-stick spray. Set aside.
Place the shortbread cookies in a large Ziplock bag. Seal the bag shut. Crush the cookies into small crumbs with a rolling pin. Pour the cookie crumbs into the prepared pan. Pour melted butter all over the crumbs and mix well. Press evenly into the pan. Freeze for 20 minutes.
Meanwhile, mix the softened vanilla ice cream with half of the chopped TWIX bars. Spread evenly over the frozen crust. Sprinkle the remaining chopped TWIX bars evenly over the top of cake. Cover the baking dish and freeze for at least 6 hours or overnight.
Once cake is frozen, and ready to serve, place the milk chocolate chips in a microwave-safe dish. Cook in 20 second increments until melted. Drizzle the milk chocolate evenly over the ice cream cake. Drizzle the caramel topping all over the cake. Slice into squares and serve!