Place a rack in the center of the oven and preheat to 350°F Line 24 cupcake cups with paper liners. Set the pans aside.
Break the chocolate bars into quarters, stack them on top of each other, and slice down the short end of the stack with a sharp knife. You will have about 1 cup of 1/4 inch shavings. Set them aside.
Place the cake mix, cream cheese, oil, eggs, and vanilla in a large mixing bowl. Blend with an electric mixer on low speed for 30 seconds. Stop the mixer and scrape down the sides of the bowl with a rubber spatula. Increase the mixer speed to medium and blend for 2 minutes more, scraping down the sides again if needed. Stir in the chocolate shavings.The batter should look thickened and smooth. Spoon or scoop 1/3 cup batter into each lined cup, filling it three quarters of the way full. Place the pans in the oven.
Bake the cupcakes until they spring back when lightly pressed with your finger 20 to 23 minutes. Remove the pans from the oven and place them on wire racks to cool for 5 minutes .