Heat oven to 350. Grease bottom and sides of 13 x 9 inch pan with shortening or spray with cooking spray.
In medium bowl, beat all filling ingredients with spoon until smooth; set aside.
In 2 quart saucepan, melted chocolate and butter over low heat, stirring frequently, until smooth; remove from heat. Cool 5 minutes.
In large bowl, beat chocolate mixture, 2 cups sugar, the vanilla and 4 eggs with mixer on medium speed 1 minute, scraping bowl occasionally. Beat in flour and salt on low speed 30 seconds, scraping bowl occasionally. Beat on medium speed 1 minute. Stir in nuts.
Spread half of batter in pan. Spread filling over batter. Carefully spread remaining batter over filling. Gently swirl through batters with knife for marbled design.
Bake 45-50 minutes or until toothpick comes out clean. Cool completely, about 1 hour. FOr brownies, cut into 8 rows by 6 rows. Store covered in refrigerator.