For best results, use natural peanut butter for a more intense flavor, though creamy commercial brands work well too.
If you don't have buttermilk, you can substitute 1/2 cup milk with 1 teaspoon lemon juice or vinegar, let it sit for 5 minutes before using.
The cake should be iced while still hot to allow the frosting to melt slightly and soak into the top layer, creating a moist finish.
Store leftovers covered at room temperature for up to 2 days, or in the refrigerator for up to 5 days. Bring to room temperature before serving for best texture.
Feel free to adjust the amount of milk in the icing to achieve your desired consistency.