Spread half of vanilla wafer crumbs in a 10- x 6- x 2 - inch baking dish. Set aside.
Combine butter, chocolate syrup, and powdered sugar in a large bowl; mix well. Stir in marshmallows, pecans, vanilla, and salt.
Beat egg whites (at room temperature) until soft peaks form. Fold into marshmallow mixture. Spread over vanilla wafer crumb layer. Top with remaining vanilla wafer crumbs. Refrigerate until thoroughly chilled or overnight. Top with sweetened whipped cream; spoon into serving dishes to serve.
Note: One 6 1/4-ounce package miniature marshmallows is equivalent to 1/2 pound large marshmallows, chopped.