Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large mixing bowl, combine the chocolate cake mix, milk, melted butter, and vanilla extract. Stir until just combined; do not overmix.
Pour the batter into the prepared baking dish, spreading it evenly.
Drop spoonfuls of peanut butter over the cake batter. Use a knife or skewer to swirl the peanut butter into the batter gently.
Sprinkle the chocolate chips and chopped peanut butter cups evenly over the top of the batter.
Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cake to cool for about 10 minutes before serving. Dust with powdered sugar if desired.