Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large mixing bowl, combine the chocolate cake mix, water, vegetable oil, and eggs. Mix until well combined and smooth. Pour the batter into the prepared baking dish.
In another bowl, beat the softened cream cheese, powdered sugar, and peanut butter together until creamy and smooth. Drop spoonfuls of the peanut butter mixture over the chocolate cake batter. Use a knife to swirl the peanut butter mixture into the cake batter gently.
Sprinkle the semi-sweet chocolate chips and chopped Reese's Peanut Butter Cups evenly over the top of the cake.
Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Remove from the oven and let cool for at least 15 minutes before serving.