Preheat oven to 350 degrees F (175 degrees C). Line two 12-cup muffin tins with paper liners, preferable dark-colored or Halloween-themed ones.
Combine cake mix, water, eggs, and oil in a large bowl; beat with an electric mixer on low speed until moistened, about 30 seconds. Beat at medium speed until batter is smooth and creamy, about 2 minutes. Spoon batter into the prepared muffin cups, filling each 3/4 full.
Bake in the preheated oven until a toothpick inserted in the center comes out clean, 18 to 21 minutes. Remove from oven and cool tins on a wire rack for 15 minutes. Remove cupcakes from the tins and cool completely on a wire rack before decorating, about 1 hour.
Frost each cupcake with a thin layer of white vanilla frosting and refrigerate for 30 minutes to make decorating easier.
Remove cupcakes from fridge and apply a second layer of white frosting. Fill a piping bag outfitted with a small round tip with chocolate frosting and draw a skull face on the cupcakes: pipe large ovals for the eyes, two dots for the nostrils, and a large "stitched" smile for the mouth.