Preheat the oven according to the cake mix package directions. Grease a 9 x 13-inch baking pan.
Prepare the chocolate cake batter according to the package instructions. Stir in the vanilla extract, then pour the batter into the prepared pan.
Bake until the center is set and a toothpick inserted into the middle comes out with a few moist crumbs. Avoid overbaking.
Place the cake on a cooling rack and let it cool for about 15 to 20 minutes.
Use the rounded end of a wooden spoon to poke evenly spaced holes across the cake. Make the holes deep enough to hold the filling without reaching the bottom of the pan.
Stir together the chocolate fudge sauce and sweetened condensed milk until smooth.
Spoon the chocolate mixture over the cake, concentrating on the holes. Gently spread the remaining filling across the surface.
Allow the cake to cool completely. Refrigerate it briefly if needed to speed up the cooling process.
Beat the peanut butter, powdered sugar, and vanilla until smooth. In a separate chilled bowl, whip the cold heavy cream until soft to medium peaks form.
Gently fold the whipped cream into the peanut butter mixture until the topping is light and creamy.
Spread the peanut butter topping evenly over the cooled cake.
Combine the chocolate chips, peanut butter, and heavy cream in a microwave-safe bowl. Heat in short intervals, stirring between each one, until smooth.
Allow the chocolate mixture to cool slightly, then drizzle it over the peanut butter topping.
Add chopped peanuts, mini chocolate chips, chocolate shavings, or chopped peanut butter cups if desired.
Cover the cake and refrigerate for at least 2 hours before serving.
Slice the chilled cake into squares and serve.