01 - Preheat oven to 350°F. Generously grease a 9x13-inch baking pan with cooking spray or butter.
02 - Prepare chocolate cake batter according to package instructions. Pour into prepared pan and bake for 30-35 minutes until toothpick inserted in center comes out clean.
03 - Let cake cool for 10 minutes. Using wooden spoon handle, poke holes evenly across surface at 1-inch intervals.
04 - Whisk instant vanilla pudding mix and cold milk in medium bowl for 2 minutes until thickened. Stir in peanut butter until completely smooth.
05 - Pour peanut butter pudding mixture evenly over warm cake, using spatula to spread and fill all holes. Refrigerate covered for 1 hour.
06 - Beat heavy cream, powdered sugar, and vanilla extract in large bowl until stiff peaks form. Spread whipped cream evenly over chilled cake.
07 - Microwave chocolate chips, peanut butter, and heavy cream for 30 seconds. Stir until completely smooth, microwaving additional 10 seconds if needed.
08 - Drizzle chocolate mixture over whipped cream layer. Sprinkle with chopped peanuts and halved peanut butter cups if using.
09 - Refrigerate for 30 minutes before serving to allow flavors to meld and drizzle to set.