Preheat the oven to 350F. Grease the inside of an 8.5 by 4.5-inch loaf pan with butter or spray it with cooking spray.
Add the peanut butter and brown sugar to a large bowl. Use a handheld electric mixer to beat until combined. Beat in the eggs, milk, and vanilla until smooth.
Add the flour, baking powder, and salt, and beat until incorporated, being careful not to over-mix.
Transfer the batter to the prepared pan and bake until a toothpick inserted in the center comes out clean, about 55 to 65 minutes.
Cool in the pan for 30 minutes, and then remove the loaf and let it finish cooling on a wire rack before slicing.
Notes
You can use 3/4 cup granulated white sugar instead of brown sugar if desired.
I would recommend sticking to regular creamy peanut butter (such as Skippy or Jif) for this recipe. Natural butters tend to be too thin.
Transform this peanut butter bread into a chocolate PB bread by stirring in 1 cup of semi-sweet chocolate chips to the batter.
Store the cooled peanut butter bread in an air-tight container kept at room temperature for 3-4 days or in the refrigerator for up to one week.
TO FREEZE - Wrap the loaf a few times in plastic wrap followed by a layer of aluminum foil. Place the loaf in a large freezer-safe bag or air-tight container and freeze for up to 3 months. You can also freeze the individual slices by flash-freezing them on a baking tray. Once solid, transfer them to a large zip-top bag and freeze. Thaw at room temperature. Flash-freezing them will help them not stick to each other, if preferred, you can wrap the individual slices in plastic wrap then store in a ziptop bag.