Melt butter and chocolate square in a medium saucepan over low heat, stirring occasionally; stir in sugar. Add eggs, 1 at a time, stirring well.
Stir in flour and next 3 ingredients. Spread into a greased and floured 13- x 9-inch baking pan. Spread Cream Cheese Filling on top; sprinkle evenly with chocolate morsels.
Bake at 350° for 30 minutes. Sprinkle with marshmallows; and bake 3 to 5 more minutes or until marshmallows melt. Pour Chocolate Frosting over marshmallows; cool completely. Cut into squares.