In a large bowl, combine cake mix, egg whites, coconut milk, and oil. Beat at low speed with an electric mixer until combined. Increase speed to medium, and continue beating for 2 minutes. Stir inn melted chocolate. Divide batter evenly among prepared muffin cups, filling about two-thirds full. Bake for 18 to 20 minutes, or until a wooden pick inserted in center comes out clean. Let cool in pans for 10 minutes. Remove from pans, and cool completely on wire racks.