In medium bowl, mix nut crunch ingredients. With pastry blender (or pulling 2 table knives through ingredients in opposite directions), cut in butter until crumbly. Stir in walnuts. Spread in ungreased 13×9-inch pan. Bake about 20 minutes or until golden brown. Cool completely, about 30 minutes.
Meanwhile, in 3-quart saucepan, heat chocolate chips, marshmallows and milk over low heat, stirring constantly, just until chocolate and marshmallows are melted and mixture is smooth. Refrigerate about 30 minutes, stirring every 10 minutes, until mixture mounds slightly when dropped from a spoon.
In chilled large bowl, beat whipping cream with electric mixer on high speed until stiff. Fold chocolate mixture into whipped cream. Fold 2 cups of the nut crunch into chocolate-cream mixture.
In ungreased 13×9-inch pan, spread 1 cup of the nut crunch. Carefully spread chocolate-cream mixture over crunch. Sprinkle with remaining crunch (about 1 cup). Cover; refrigerate until set, at least 8 hours but no longer than 24 hours.