Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with non-stick spray or butter.
In a large mixing bowl, combine the chocolate cake mix, milk, melted butter, peanut butter, vanilla extract, and salt. Stir until just combined; do not overmix.
Pour the batter into the prepared baking dish and spread it evenly.
Sprinkle the semi-sweet 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 or with a few moist crumbs.
Remove from the oven and let cool for about 10-15 minutes before serving.