- 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.