1. Heat oven to 350 degrees F. Coat a large baking sheet with nonstick cooking spray; set aside.
2. Blend flour, walnuts and salt in a bowl.
3. In second bowl, beat together butter, sugar and vanilla for 2 minutes or until creamy and smooth. Stir in flour mixture.
4. Roll 1 tablespoon dough into a ball; place on prepared baking sheet. Repeat with remaining dough. Gently flatten each ball to a 1/2-inch thickness and smooth its edges. Bake at 350 degrees for 16 minutes or until lightly browned at edges.
5. Meanwhile, stir together marshmallows and coarsely chopped walnuts in a small bowl. Melt semisweet chocolate chips in another small bowl in microwave oven for 1 minute, stirring halfway through.
6. Remove cookies from oven and spread each with 1 teaspoon of the chocolate, then sprinkle with the marshmallow mixture. Return to the oven for 1 to 2 minutes, until the marshmallows soften. Let cool on baking sheet on wire rack for 2 minutes before transferring to wire rack to cool completely. Makes 16 cookies.