Place chocolate and butter together in a microwave-safe bowl; heat on high setting just until melted. Cool until just warm.
Blend together eggs and sugar until light and foamy. Add flour and salt; mix well. Pour in chocolate mixture; blend until smooth. Spoon batter into 12 paper-lined muffin cups; sprinkle chocolate chips evenly over tops.
Bake at 350° for 15 minutes or until toothpick tests clean. Remove from oven; arrange several marshmallows on top of each cupcake. Broil just until marshmallows turn golden. Remove from oven and let stand 5 minutes to cool slightly.