In the bowl of a stand mixer with paddle attachment, cream together room temperature butter, peanut butter, vanilla, eggs (one at a time) and brown sugar in mixer.
In a separate bowl, sift together flour, baking powder, baking soda, and salt.
Add the flour mixture to the sugar mixture, alternating with buttermilk. As in, add 1/3 flour mixture then 1/2 cup of buttermilk, another 1/3 of the flour mixture, remaining buttermilk, then remaining flour.
Bake in two prepared 9-inch round cake pans at 350°F for 25-35 minutes.