In a large mixing bowl, combine cake mix, eggs, water, and oil.
Line two regular-sized muffin pans with paper liners.
Fill muffin cups 1/2 full.
Bake at 350 degrees for 15-18 minutes, or until edges are golden brown and the tops of cupcakes spring back when pressed with a finger.
Cool in pans for about 10 minutes before removing to wire racks to cool completely.
Using a small spoon or knife, scoop a 1-inch ball out of the center of each cupcake. Discard the scooped cake balls, or repurpose (make cake balls!).
Fill each cavity evenly with Lucky Charms marshmallows.