HEAT oven to 350ºF.
BEAT butter, 1/2 cup sugar, flour and salt with mixer until mixture resembles coarse crumbs. Press onto bottom of greased 15x10x1-inch pan. Bake 20 min. or until lightly browned.
MICROWAVE corn syrup and chocolate in large microwaveable bowl on HIGH 2-1/2 min. or until chocolate is almost melted, stirring after 1-1/2 min.; stir until completely melted. Add remaining sugar, eggs and vanilla; mix well. Stir in nuts. Pour over warm crust; spread to evenly cover crust.
BAKE 35 min. or until filling is firm around the edges but still slightly soft in center. Cool completely before cutting into bars.