PREHEAT oven to 350°F. Line 9-inch square baking pan with foil, with ends of foil extending over sides of pan. Grease foil.
BEAT butter, sugar, egg and vanilla in large bowl with electric mixer on medium speed until light and fluffy. Add flour, baking soda and salt; mix well. Stir in half each of the chocolates and walnuts. Spread into prepared pan.
BAKE 25 min. or until toothpick inserted in center comes out almost clean. (Do not overbake.) Remove from oven. Sprinkle with remaining chocolates. Cover with foil. Let stand 5 min. or until chocolates are melted. Cut through chocolates with knife several times for marble effect. Sprinkle with remaining 1/2 cup walnuts. Cool in pan on wire rack. Lift dessert from pan, using foil handles. Cut into 32 bars. Store in tightly covered container at room temperature.