Preheat oven to 350 degrees F. Grease and flour a 9 inch square pan.
Sift flour, baking powder and salt together. Set aside.
In a medium size bowl, combine sugar, egg, butter, and vanilla. Beat until smooth. Stir in flour mixture. Stir in 1/2 cup nuts. Pour into jelly roll pan. Bake for 15 minutes.
Remove from oven; while hot, arrange marshmallows and remaining nuts and chocolate chips on top of bars. Return to oven for 2 minutes or just until chocolate is softened. Swirl and cool until chocolate is set. Cut into squares.
