Preheat and Prepare: Preheat your oven to 350°F (175°C) and grease a 9×13-inch baking dish.
Assemble the Cake: In the prepared baking dish, layer the chocolate cake mix evenly. Pour the milk and melted butter over the dry cake mix, spreading to cover. Scatter peanut butter chips, chocolate chips, and mini peanut butter cups evenly across the top.
Add Sweetened Condensed Milk: Pour the sweetened condensed milk evenly over the entire mixture.
Bake: Bake in the preheated oven for 30-35 minutes, or until the edges are bubbly and the center is set.
Serve: Let cool slightly, then serve warm with whipped cream or vanilla ice cream if desired.