Preheat the oven to 350 degrees and grease 9x5 loaf pan.
In a medium bowl, add flour, baking powder, baking soda and salt and whisk for about 45 seconds.
Using a mixer, in a large bowl add the peanut butter, oil, and brown sugar and beat until combined.
Add eggs and maple syrup or vanilla extract and continue beating until just combined.
Alternately add the flour and buttermilk to the wet mixture, beating just until combined and do not overbeat. (A few small lumps of flour is fine, overbeating will result in a dry, dense cake.)
Pour batter into the bread loaf pan.
Bake for about 15-18 minutes.
Let the cake sit in the pan for 5 minutes, then invert the cake onto a wire rack to cool completely.