Prep pans and oven. Preheat your oven to 350°F and prepare your cupcake pans with cupcake paper liners. Set aside.
Combine dry ingredients. In a medium mixing bowl, combine the cake flour, baking powder and salt. Whisk together and set aside.
Cream butter. Add the butter to the bowl of an electric mixer fitted with the paddle attachment. Beat on medium-high speed for 3 minutes, until light and creamy in color. Scrape down the sides of the bowl and beat for one more minute.
Add sugar. Add the sugar to the butter mixture, ¼ cup at a time, beating 1 minute after each addition.
Add eggs and peanut butter. Mix in the eggs one at a time until incorporated. Add in your peanut butter and mix to combine. Scrape down the sides of the bowl as needed.
Combine buttermilk and vanilla. Combine the buttermilk and the vanilla extract in a liquid measuring cup and whisk to combine.
Alternate dry ingredients with buttermilk. With the mixer on low speed, add the dry ingredients alternately with the buttermilk/vanilla mixture, beginning and ending with the dry ingredients and mixing just until incorporated. Scrape down the sides of the bowl and mix for 15 seconds longer.
Bake cupcakes. Divide the batter between the prepared paper liners, filling each about 2/3 of the way full. Bake 18-22 minutes, or until a toothpick inserted in the center comes out clean. Allow to cool in the pans 5-10 minutes, then transfer to a cooling rack to cool completely.