01 - Preheat oven to 350°F and generously grease a 9x13-inch baking pan with cooking spray or butter.
02 - Prepare chocolate cake mix according to package directions. Pour batter into prepared pan and bake for 30-35 minutes until a toothpick inserted in center comes out clean.
03 - Let cake cool for 10 minutes. Using handle of wooden spoon, poke holes all over cake at 1-inch intervals, pressing about three-quarters of the way through.
04 - Whisk instant vanilla pudding mix and cold milk in medium bowl for 2 minutes until slightly thickened. Stir in peanut butter until completely smooth and combined.
05 - Pour peanut butter pudding evenly over warm cake, using spatula to spread filling into all holes. Cover and refrigerate at least 1 hour until set.
06 - Beat heavy cream, powdered sugar, and vanilla extract in large bowl until stiff peaks form. Spread whipped cream evenly over chilled cake surface.
07 - Combine chocolate chips, peanut butter, and heavy cream in microwave-safe bowl. Microwave for 30 seconds, stir until smooth. Microwave additional 10 seconds if needed.
08 - Drizzle warm chocolate-peanut butter mixture over whipped cream layer. Sprinkle with chopped peanuts and halved mini peanut butter cups if desired.
09 - Refrigerate completed cake for 30 minutes before serving to allow flavors to meld and layers to set properly.