Preheat the oven to 350°F, and grease a 9 x 5-inch loaf pan.

In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a separate large bowl, use a hand mixer to combine the brown sugar and peanut butter until smooth. Add the egg, and continue to mix until well incorporated. 

Alternate adding half dry ingredients and buttermilk into the peanut butter mixture. Mix the ingredients on low speed just until combined after each addition.

Pour the batter into the prepared loaf pan, and bake for 40 to 50 minutes or until a toothpick inserted in the center comes out clean. About halfway through the cook time, it may be necessary to fold a piece of aluminum foil in half and place it loosely over the top to prevent over browning.

After the bread bakes, allow it to cool for 5 to 10 minutes in the pan before transferring it to a wire rack. Cool completely before slicing. Store in an airtight container, at room temperature, for up to 3-days.