Preheat oven to 350 degrees. Grease a 9 x 5 x 3-inch loaf pan, set aside.
In a large bowl, using an electric mixer, beat the peanut butter, butter, and sugars together. Add the egg and vanilla, beat until combined.
In a separate bowl, mix together the flour, baking powder, and salt. Add the dry ingredients to the peanut butter mixture. Beat together until combined (the mixture will be very crumbly). Pour in milk and beat until smooth.
Spoon batter into a prepared loaf pan. Bake for 50 minutes or until the center springs back when touched.
Cool in the pan on a wire rack for 15 minutes. Remove from pan, cool completely, then wrap in foil and store in a cool place.
If using unsalted butter, increase the salt to 1 teaspoon.