Heat oven to 350°F (325°F for dark or nonstick pans). Place paper baking cup in each of 12 regular-size muffin cups.
In large bowl, beat cake mix, water, oil and egg with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Divide batter evenly among muffin cups.
Bake 17 to 22 minutes or until toothpick inserted in center comes out clean. Cool in pan 10 minutes; remove from pan to cooling rack. Cool completely, about 30 minutes.
In small bowl, mix frosting and marshmallow creme. Spoon into small resealable food-storage plastic bag; seal bag. Cut 3/8-inch tip off 1 corner of bag. (Or spoon mixture onto cupcakes instead of piping.)
Pipe 3 small dollops of frosting mixture on top of each cupcake to look like melted marshmallows. Sprinkle with cocoa. Press pretzel half into side of each cupcake for cup handle. Store loosely covered.