Heat oven to 350 degrees Fahrenheit. Place paper baking cup in each of the 24 regular sized muffin cups. Microwave 4 oz of the white chocolate on high 30 to 60 seconds, stirring once until chocolate can be stirred smooth; set aside.
Make cake mix as directed on box. Stir in melted chocolate. Divide batter evenly among muffin cups. Bake as directed. Cool 5 minutes; remove from pans to cooling racks. Cool completelyl
Microwave remaining 10 ounces of white chocolate 1 minute and 30 seconds stirring after 30 seconds, until it stirs smooth. Dip tops of cupcakes in chocolate. Arrange on a cookie sheet. Refrigerate until set, about 10 minutes.
Set oven to broil. Line cookie sheet with parchment paper. Place marshmallows, cut sides down on cookie sheet. Broil 1 to 2 minutes until golden brown. Let stand 2 to 3 minutes. With metal spatula place marshmallow on cupcakes. Sprinkle with cocoa.