Preheat oven to 350°. Melt unsweetened chocolate and butter together. Add sugar and cool mixture. Add eggs, beating well, one at a time. Add vanilla, flour, and salt. Mix until well blended. Fold in chocolate chips and marshmallows. Pour into a greased 13 x 9 inch baking pan. Bake at 350° for 30-35 minutes. Do not overbake. The top will look bubbly.
Remove from oven and dust well with powdered sugar. Allow to cool thoroughly before slicing since they are really gooey. Cut into small squares and freeze (we put them in square Rubbermaid containers and layered them with plastic wrap).
These keep several months in the freezer and are an excellent frozen desert.
From Gateways, St. Louis Children's Hospital, 1999.