Ghirardelli Mississippi Mud Bars

1/2 c butter or margerine, softened
3/4 c brown sugar, firmly packed
1 tsp vanilla extract
1 large egg
1 c all purpose flour
1/2 tsp. baking soda
1/4 tsp salt
1 bar (4 oz) semi-sweet baking bar, chopped and divided
1 bar (4 oz) white chocolate baking bar, chopped and divided
1 c walnuts, chopped and divided

Preheat to 350. In a large bowl. beat butter with sugar at medium speed until light and fluffy (about 4 minutes). Add vanilla and egg, mix on low speed until incorporated. combine flour, baking soda, and salt; gradually add to creamed mixture. Mix in 1/2 of the chopped semi-sweet and white chocolate bars and 1/2 c walnuts. Spread in a greased 9" square baking pan. Bake for 12-15 min or until toothpick inserted in center comes out almost clean. Sprinkle with remaining semi-sweet and white chocolates. Cover with foil. Let stand 5 minutes or until chocolates are melted. Swirl chocolates with small knife to marbleize. Sprinkle with remaining 1/2 c walnuts. Cool in pan until chocolate is firm. Cut into bars. Makes 1 doz bars.