Mix 1/2 cup butter, the sugar, evaporated milk and the chocolate in a pan. Bring mixture to a boil over moderate heat. Stir constantly. Boil until the mixture has thickened. Refrigerate it until it's completely cooled. Mix the melted butter and the Oreos in a bowl. Press mixture over bottom of a baking dish, 9 x 13 inches. Spread the ice cream over the cookie layer. Spread the chilled chocolate mixture over the ice cream. Next, spread the whipped topping over the layers and sprinkle the pecans over the top. Cover and freeze for 8 to 12 hours.