1. Preheat oven to 350°F. Grease bottom of rectangular pan, 13x9x2 inches, with shortening or cooking spray. Melt chocolate and butter over low heat, stirring frequently, until smooth; remove from heat. Cool 5 minutes.
2. Meanwhile, beat all ingredients of Peppermint Cream Cheese Filling together with spoon until smooth. Set aside.
3. Beat chocolate and butter mixture with sugar, vanilla and eggs in large bowl with electric 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. Spread half of batter (about 2 1/2 cups) in pan. Spread filling over batter. Carefully spread remaining batter over filling. Gently swirl through batter with knife for marbled design.
4. Bake 45 to 50 minutes or until toothpick inserted in center comes out almost clean. Cool completely.
5. Chocolate Frosting. Heat butter, corn syrup and water to boiling in 1-quart saucepan; remove from heat. Add chocolate, stirring until melted. Stir in enough powdered sugar until spreadable.