Step 1: Preheat your oven to 350 °F (175 °C) and thoroughly grease a 9x13-inch baking dish with nonstick spray or a thin layer of butter. Ensure the oven rack is centered for even heat distribution.
Step 2: Spread the creamy peanut butter in an even layer across the bottom of the prepared dish, making sure it reaches all corners to create a moisture barrier.
Step 3: Evenly distribute the dry chocolate cake mix atop the peanut butter. Gently shake the pan or use a spoon to fill any gaps without compressing the powder.
Step 4: Slowly pour the sweetened condensed milk over the surface of the cake mix, aiming to cover it uniformly. Do not stir or mix the layers.
Step 5: Place thin pats of unsalted butter across the cake mix in a grid pattern. Ensure each slice touches the mix to promote even melting and golden browning.
Step 6: Gently press the halved peanut butter cups into the top layer, spacing them evenly to distribute the chocolate-peanut pockets throughout the cake.
Step 7: Sprinkle the chopped roasted peanuts (if using) between the peanut butter cups to add a crunchy texture and enhance the nutty aroma.
Step 8: Transfer the dish to the preheated oven and bake for 35 to 40 minutes, or until the edges are golden brown and the center appears just set. A toothpick inserted near the center may show moist crumbs, but no wet batter.
Step 9: Remove from the oven and let the dump cake rest on a cooling rack for at least ten minutes. This allows the layers to set, making for cleaner slicing.