Beat 3/4 cup cream cheese at medium speed with an electric mixer until smooth. Add 1/2 cup sugar, 2 tablespoons flour, 1/4 cup butter, 1 egg, and 1/2 teaspoon vanilla; beat until blended. Set cream cheese batter aside.
Microwave 1/2 cup butter and 1 square unsweetened chocolate in a 1-quart microwave-safe bowl at HIGH 1 minute or until melted, stirring once.
Beat 2 eggs at medium speed, and gradually add remaining 1 cup sugar, beating well. Add 1 teaspoon vanilla extract, and beat until blended. Add 1 cup flour and baking powder, beating well. Add melted chocolate mixture, and beat until chocolate batter is blended. Stir in 1 cup chopped walnuts.
Pour chocolate batter into a greased 13- x 9-inch pan. Top with cream cheese batter. Sprinkle with remaining 1/4 cup chopped walnuts and chocolate morsels.
Bake at 350° for 28 minutes. Sprinkle with marshmallows, and bake 2 more minutes.
Combine remaining 1/4 cup butter, remaining 1/4 cup cream cheese, remaining 1 square unsweetened chocolate, and milk in a saucepan. Cook over medium heat, stirring constantly, until butter and chocolate melt. Stir in powdered sugar and remaining 1 teaspoon vanilla. Drizzle over brownies. Chill; cut into squares.