As the stock is heating, in a separate large saucepan, heat butter and 2 tablespoons of olive oil. When hot, add the chicken breasts and seal all around until some of the flesh is browning slightly. Season the breast with a little salt and pepper as they cook. Remove the pan from the heat and remove the chicken breasts, placing them straight into the simmering stock. Retain butter, oil and chicken juices in the pan.