PREHEAT oven to 350°F. Line 13x9-inch baking pan with foil, with ends of foil extending over sides of pan. Lightly grease foil; set aside. Beat 3/4 cup each of the butter and brown sugar in large bowl with electric mixer on medium speed until light and fluffy. Blend in egg yolk. Add flour and salt; mix well. Place in prepared pan; press firmly onto bottom to form crust. Bake 16 to 18 min. or until golden brown.
MEANWHILE, place remaining 1/4 cup each butter and brown sugar, the corn syrup, whipping cream and vanilla in large microwaveable bowl. Microwave on HIGH 4 min., stirring every 2 min.; set aside. Cool crust slightly on wire rack. Spread butter mixture evenly over crust.
BAKE an additional 18 to 20 min. or until set. Sprinkle with chopped chocolate. Continue baking 1 to 2 min. or until chocolate is melted; spread chocolate evenly over dessert. Sprinkle with pecans. Cool completely. Refrigerate 30 min. or until chocolate is firm. Remove dessert from pan, using foil handles. Cut into 36 bars or diamond shapes. Store in tightly covered container at room temperature.