1Heat oven to 350°F. Spray bottom and sides of 9-inch square pan with cooking spray.
2In medium bowl, beat 1/2 cup butter, the granulated sugar and egg yolk with electric mixer on low speed 30 seconds, scraping bowl occasionally. Add flour and salt; beat until moist clumps form. Press dough in bottom and 1/2 inch up sides of pan. Bake 15 to 18 minutes or until golden brown. Cool 5 minutes.
3In 2-quart saucepan, heat syrup, brown sugar, whipping cream and 3 tablespoons butter to boiling over medium heat, stirring constantly, until butter is melted and mixture is smooth. Remove from heat; stir in vanilla and walnuts. Pour hot filling over partially baked crust.
4Bake 15 minutes or until filling is bubbling in center. Cool completely in pan on cooling rack. Drizzle with melted chocolate. Refrigerate 1 hour. Cut into 4 rows by 4 rows.