Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large bowl, combine the chocolate cake mix, water, vegetable oil, and eggs until well blended. Pour the batter into the prepared baking dish.
In another bowl, mix together the creamy peanut butter, softened cream cheese, melted butter, powdered sugar, and vanilla extract until smooth.
Drop spoonfuls of the peanut butter mixture over the top of your chocolate batter and swirl gently with a knife.
Sprinkle chopped Reese’s Peanut Butter Cups over the top and bake for 35-40 minutes or until a toothpick comes out mostly clean.
Allow the cake to cool slightly before cutting into squares. Enjoy warm or at room temperature.