Preheat the oven to 375°F and line a cupcake pan with liners, and set aside.
In a large mixing bowl or stand mixer fitted with a whisk attachment, combine 1 (15.oz.) box Duncan Hines Dark Chocolate Fudge Cake Mix, 1 (3.4oz.) box Jell-O Instant Chocolate Pudding, 4 large eggs, 1 cup sour cream, ¾ cup vegetable oil, ½ cup whole milk, 1 tablespoon vanilla extract, ¼ teaspoon salt, making sure to scrape down the sides as needed.
Add a scant ¼ cup of batter to each liner so they’re about ¾ full.
Reduce heat to 350°F and bake the cupcakes for about 18 to 20 minutes or until a toothpick inserted in the center of a cupcake comes out clean.
Let the cupcakes sit in the pan for a couple of minutes before transferring to a cooling rack to cool completely before frosting.
Use a cupcake corer to core the cupcakes, and divide 1 bag mini Reese's Pieces1 bag mini Reese's Pieces to fill the holes with.