Heat oven to 325 degrees F. Coat 8 x 8 x 2-inch baking pan with nonstick cooking spray.
Combine flour and salt in small bowl.
Beat butter and brown sugar in large bowl until smooth and creamy. Beat in egg and vanilla. Stir in flour mixture and chopped walnuts.
Evenly pat into prepared pan. Bake at 325 degrees F for 35 to 40 minutes or until center is firm to the touch. Cool pan on wire rack.
Meanwhile, place white chocolate in small plastic food-storage bag. Leave top open. Microwave on high 30 seconds. Press chocolate to soften. If not melted, microwave another 30 seconds. Snip one corner of bag. Drizzle white chocolate on top. Cut into squares. Makes 16 squares.