Preheat Oven: Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking pan with butter or cooking spray.
Mix Wet Ingredients: In a large mixing bowl, whisk together peanut butter, milk, vegetable oil, eggs, and vanilla extract until smooth and well combined.
Add Cake Mix: Add the chocolate cake mix to the wet ingredients. Stir gently with a whisk or fork until just combined. Do not overmix.
Fold in Mix-Ins: Add chocolate chips and chopped peanut butter cups to the batter. Fold gently to distribute throughout.
Pour and Spread: Pour the batter into the prepared pan. Spread evenly with a spatula.
Bake: Bake for 45 minutes or until a toothpick inserted in the center comes out with just a few moist crumbs.
Cool and Serve: Let the cake cool for 10 minutes. Serve warm, optionally dusted with powdered sugar. Top with ice cream or whipped cream if desired.