1Heat oven to 350°F. Place paper baking cup in each of 24 regular-size 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.
2Make cake mix as directed on box, using water, oil and eggs. Stir in melted white chocolate. Divide batter evenly among muffin cups. Bake as directed. Cool 5 minutes; remove from pans to cooling racks. Cool completely.
3Microwave remaining 10 oz white chocolate on High 1 minute 30 seconds, stirring after 30 seconds, until chocolate can be stirred smooth. Dip tops of cupcakes in melted chocolate. Arrange dipped cupcakes on cookie sheet. Refrigerate until set, about 10 minutes.
4Set oven control to broil. Line cookie sheet with cooking parchment paper. Place marshmallows, cut sides down, on cookie sheet. Broil 1 to 2 minutes or until golden brown. Let stand 2 to 3 minutes. With metal spatula, slide 1 toasted marshmallow half onto each cupcake. Sprinkle with cocoa.