Preheat oven to 350 degrees. Line with foil and grease an 8 or 9 inch square baking dish. Beat together butter, brown sugar, egg and vanilla until light and fluffy. Add in flour baking soda and salt. Mix well. Sir in half of the chocolate, white chocolate chips and walnuts. Bake 25 minutes until a toothpick comes out clean. Immediately remove from oven and sprinkle remaining chips. Cover with foil and allow to melt. Swirl chocolates with a knife to marbleize. Sprinkle remaining walnuts and cool in pan. Cut into bars.
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