Where did you determine that "Qf6 is a perfectly acceptable book opening". I highly doubt that. Even if it were, the best response would be Nc3 (threatening Nd5, where ...c6 cannot be played), not Bxc6.
Ruy Lopez exercise
If you need help, please contact our Help and Support team.
So, I decided to go ahead and move Bxc6 on the fourth move...