Slice each chicken breast in half horizontally. Place each half between sheets of plastic wrap and pound with a meat mallet (or small skillet) until 1/4 inch (6 mm) thick or less.
Generously season chicken with salt and pepper.
In a large skillet heat 2 tablespoons (30 ml) of olive oil. Add half of the chicken and cook for 3 minutes on each side until nicely browned. Remove the chicken from the skillet and set aside on a plate.
Heat 2 more tablespoons of olive oil. Add the remaining raw chicken and brown for 3 minutes on each side.
Remove the chicken from the pan, adding it to the plate of cooked chicken, but keep the heat on under the skillet.
To the skillet, add 3 tablespoons of brine from the jar of capers.
As the brine boils, scrape up any bits of meat stuck to the pan.
Stir in the butter, parsley and capers. Just before the butter begins to brown, add the chicken and any juices that have accumulated on the plate back to the skillet. Keep the chicken in the skillet just long enough to re-heat it and coat the breasts in sauce.