1Heat oven to 350°F (325°F for dark or nonstick pan). Place paper baking cup in each of 24 regular-size muffin cups.
2In large bowl, beat cake mix, water, oil and eggs with electric mixer on low speed 30 seconds, then on medium speed 2 minutes, scraping bowl occasionally. Gently stir in sprinkles. Divide batter evenly among muffin cups, filling each about two-thirds full.
3Bake as directed on box for cupcakes 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.
4Frost cupcakes with frosting. Sprinkle marshmallows on frosted cupcakes; press lightly into frosting. Store loosely covered.