Preheat the oven: Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan or line it with parchment paper.
Prepare the cake batter: In a large mixing bowl, combine the yellow cake mix, eggs, softened butter, water, and peanut butter. Using an electric mixer, beat on medium speed for about 2 minutes until the batter is smooth and well combined.
Bake the cake: Pour the batter into the prepared baking pan and smooth the top. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool completely in the pan.
Make the frosting: In a medium bowl, beat together the confectioners' sugar, peanut butter, and softened butter until smooth. Add the vanilla extract and 2 tablespoons of milk, beating until you achieve a smooth and spreadable consistency. If needed, add more milk, one tablespoon at a time, until the frosting reaches your desired texture.
Frost the cake: Once the cake has completely cooled, spread the peanut butter frosting evenly over the top of the cake using a spatula.
Serve: Slice and serve the cake immediately, or store it in the refrigerator for later. Enjoy the rich, peanut buttery goodness!
