Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.

In a large mixing bowl, combine the softened butter and creamy peanut butter. Beat on medium speed until smooth and fluffy.

Gradually add granulated sugar while continuing to beat. Once combined, add in the eggs one at a time, mixing thoroughly after each addition.

In another bowl, whisk together flour, baking soda, baking powder, and salt.

Alternate adding the dry mixture and buttermilk to the peanut butter mixture. Stir in vanilla extract until well blended.

Divide the batter evenly between the prepared cake pans, smoothing the tops.

Bake for about 30 minutes, or until a toothpick inserted in the center comes out clean. Allow to cool in pans for 10 minutes before transferring to wire racks.

In a medium bowl, combine softened butter, peanut butter, and powdered sugar. Add milk and vanilla extract, whipping until smooth and creamy.

Once the cakes are completely cool, place one layer on a serving plate, slather it with frosting, and add the second layer.

Top off the entire cake with the remaining frosting and add any desired toppings.