Beat (6 ounces) 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.
In another bowl, beat 2 eggs at medium speed, and gradually add remaining 1 cup sugar, beating well.
Add 1 teaspoon vanilla extract to the egg/sugar mixture, and beat until blended.
Add 1 cup flour and baking powder to the egg/sugar/vanilla mixture, beating well.
Add melted chocolate to the egg/sugar/vanilla/flour 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 the one cup of chocolate chips.
Bake at 350° for 24-28 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. (If frosting is too thick, add a tad more milk until desired spreading consistency).
Drizzle or spread over brownies.
Chill; cut into squares.