Preheat oven to 350°F. Prepare chocolate cake batter according to package instructions. Pour into a 23x33 cm baking dish and bake as directed. Cool for 10 minutes.
Use the handle of a wooden spoon to poke holes about 2 cm apart across the warm cake surface. Let the cake cool further while preparing the filling.
Microwave peanut butter in 15-second intervals, stirring until melted and smooth. Stir in sweetened condensed milk until combined.
In a large bowl, whisk vanilla pudding mix and milk until smooth but not thick. Quickly whisk in the peanut butter mixture until blended.
Fill a piping bag or ziplock bag with half of the peanut butter filling. Pipe filling generously into each hole. Spread the remaining filling evenly over the cake surface.
Refrigerate the cake for 2 hours until the filling is set.
Beat cream cheese and vanilla extract until smooth. Add powdered sugar and one-third of the heavy cream; beat on low until combined.
Gradually add remaining heavy cream, increase speed to medium-high, and whip until stiff peaks form.
Spread the cream cheese whipped topping evenly over the chilled cake, smoothing the surface.
Melt peanut butter and drizzle over the frosted cake. Use a skewer to create a marbled effect.
Garnish with chopped Reese’s Cups and Reese’s Pieces. Store covered in the refrigerator.