Heat the oven and prepare the pan. Preheat oven to 350 degrees. Spray an 8 1/2 x 4 1/2 inch loaf pan with nonstick cooking spray.
Prepare the batter. Whisk together the flour, baking powder, baking soda, and salt. Set aside.
Beat together the brown sugar, butter, and peanut butter on medium-high speed until light and fluffy. Stop the mixer once to scrape down the bottom and sides of the bowl.
Bake the Bread. Spread the batter into the prepared pan. Bake until a cake tester inserted in the center comes out clean, about 70 minutes. (Internal temperature of the bread should be 205 degrees.)