Place first 5 ingredients in ungreased 9x13 cake pan. Mix well. Make 3 holes down center of pan. Put vanilla in first hole, oil in second, and vinegar in third. Pour the 2 cups cold water over all, and fold with fork until well blended do not beat.
Bake at 350 degrees for 40 minutes.
After cake cools make frosting.
Melt chocolate squares and butter together till hot. Stir powdered sugar, milk, and vanilla together then add to melted chocolate and butter. Add miniature marshmallows stir, and pour over cake DO NOT LET FROSTING COOL