1. Preheat oven to 350 degrees F. Line 8-inch square baking pan with parchment paper or nonstick foil.
2. Put wafer cookies in food processor and pulse until fine crumbs form. (Or place cookies in large zip-close plastic bag and crush with rolling pin). Transfer crumbs to small bowl; stir in butter and salt until crumbs are evenly moistened. Transfer to prepared pan, pressing to form even layer.
3. Sprinkle chocolate chips and walnuts over crust. Pour condensed milk over and sprinkle with marshmallows. Bake until chocolate is melted and marshmallows begin to brown, about 15 minutes.
4. Let cook in pan on a wire rack. Refrigerate until firm. Cut bars into 4 strips, the cut each strip crosswise into 4 pieces, making total of 16 bars.