Heat chocolate and butter in a saucepan over medium-high heat, stirring constantly, until melted. Stir in sugar, and cook, stirring constantly, until sugar melts. Cool 15 minutes.
Add eggs; stir until blended. Add flour, salt, and vanilla, stirring until blended. Stir in chocolate morsels and marshmallows, and pour into a greased 13- x 9-inch pan.
Bake at 350° for 30 to 35 minutes; cool in pan on a wire rack. Cover brownies, and let stand at room temperature 8 hours.