Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish. In a mixing bowl, combine the chocolate cake mix, eggs, water, vegetable oil, and vanilla extract. Blend until smooth.

Pour the cake batter into the greased baking dish, spreading it evenly. Bake for 25–30 minutes, checking for doneness with a toothpick. Let the cake cool for 10 minutes.

Prepare the peanut butter filling by whisking together peanut butter, sweetened condensed milk, and whole milk until smooth.

Poke holes in the warm cake using the handle of a wooden spoon, about 1 inch apart.

Pour the peanut butter filling over the poked cake, ensuring it seeps into the holes. Spread any excess filling on top.

In a mixing bowl, combine heavy cream and powdered sugar. Beat until soft peaks form.

Gently fold in peanut butter into the whipped cream until smooth.

Once the cake is cooled, spread the peanut butter whipped topping evenly across the top.

Sprinkle chopped Reese's cups and crushed peanuts on top, drizzling with melted chocolate and peanut butter.

Cover with plastic wrap and refrigerate for at least 1 hour before serving.