Heat the oven to 350°F (175°C) and grease a 9x13-inch baking dish. In a large bowl, combine the cake mix, instant chocolate pudding mix, sour cream, eggs, and neutral oil.

Stir the mixture just until evenly blended, taking care not to overmix. The batter will be very thick, which is exactly what creates the cake’s dense, fudgy texture.

Fold about half of the chocolate chips and half of the peanut-butter chips into the batter. Spread the mixture evenly in the prepared baking dish.

Scatter the remaining chocolate chips and peanut-butter chips over the surface. Add the chopped peanut-butter cups, distributing them so every portion will have plenty of candy.

Bake for approximately 40–50 minutes, until the center is mostly set and a tester comes out with moist crumbs rather than wet batter. Remove the cake while the center still looks slightly underdone so it stays fudgy as it cools.

Cool the cake for at least 20–30 minutes before serving. For especially clean slices, cool it completely or refrigerate it for 30–60 minutes, then cut and serve.