PREHEAT oven to 350° F. Grease 8-inch-square baking pan.
MELT butter and 1/3 cup morsels in medium saucepan over medium heat, stirring constantly, until melted (about 4 to 5 minutes). Remove from heat. Stir in sugar, flour, nuts, eggs and vanilla extract until well mixed. Spread into prepared baking pan.
BAKE for 25 to 30 minutes or until sides of bars begin to pull away from edges of pan. Sprinkle with marshmallows. Continue baking for 3 minutes.
COMBINE 1/2 cup morsels and vegetable shortening in small, microwave-safe bowl. Microwave on HIGH for 1 minute, stirring occasionally, until melted. Drizzle bars with chocolate. Cool completely in pan on wire rack. Cut into bars.