Grease the bottoms only of twelve 6-oz. ramekins or custard cups; set aside. In a large microwave, melt butter and chocolate; stir until smooth. Set aside.
In a large bowl, beat eggs and yolks on high speed for 3 minutes or until light and fluffy. Gradually add sugar, beating until thick and lemon-colored, about 5 minutes. Beat in chocolate mixture. Combine flour and baking powder; beat into egg mixture just until combined.
Fill prepared ramekins half full; sprinkle with marshmallows. Bake at 400° for 12-15 minutes or until the top is puffed and center appears set. Sprinkle with cinnamon-sugar; serve immediately. Refrigerate leftovers.