Preheat the Oven: Preheat to 350°F (175°C). Grease and flour two 8-inch round cake pans, or line them with parchment paper for easy removal.
Make the Cake Batter: Prepare the chocolate cake mix according to the instructions on the package. Stir in 1/2 cup peanut butter and 1/2 cup milk until smooth and evenly mixed. The added peanut butter makes the cake extra moist and rich. Divide the batter evenly between the two prepared pans.
Bake the Cakes: Bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean. Allow cakes to cool in the pans for 10–15 minutes before transferring them to wire racks to cool completely.
Prepare the Filling: In a large mixing bowl, beat together 1 cup peanut butter, powdered sugar, softened butter, and vanilla extract. Mix until smooth and creamy, with a light, spreadable consistency. This filling should be thick but not stiff — adjust with a touch of milk if needed.
Assemble the Cake: Place one cooled cake layer on a serving plate. Spread half of the peanut butter filling evenly over the top. Gently place the second cake layer on top and cover the top and sides with the remaining filling, smoothing it out with a spatula or offset knife.
Dust and Decorate: Generously dust the cake with powdered sugar for a classic look. For added flair, top with chopped peanuts, a drizzle of melted chocolate, or a swirl of whipped cream around the edges.
Serve: Slice and serve slightly warm for ultimate gooeyness, or refrigerate and serve chilled for firmer layers. Either way, the result is an irresistibly decadent treat!
