In medium bowl, mix crushed cookies and 1/2 cup melted butter. Press mixture firmly in bottom of ungreased 13x9-inch pan.
In large bowl, stir together ice cream and 2 cups of the whipped topping; spoon evenly over cookie crust. Freeze about 3 hours or until firm.
Meanwhile, in 2-quart saucepan, melt 1/2 cup butter and the chocolate over low heat, stirring constantly.Stir in sugar; gradually stir in milk. Heat to boiling over medium-high heat, stirring constantly. Cook and stir 5 minutes or until slightly thickened; stir in vanilla. Cool completely, about 2 hours.
Pour 2 cups of the chocolate sauce evenly over ice cream. Freeze at least 8 hours but no longer than 2 weeks. Cover and refrigerate remaining sauce.
Reheat reserved sauce until just warm. To serve, cut into 5 rows by 3 rows. Top with sauce and remaining whipped topping; sprinkle with crushed candies.