Bring a large pan of salted water to the boil, then tip in the sprouts.
Bring back to the boil and cook for 5 minutes.
Drain, run under the cold tap until cold and drain again.
Heat a large frying pan, add the pancetta and fry until crisp.
Remove the pancetta from the pan leaving any cooking oils, then add the chestnuts and fry over a high heat for about 5 minutes until browned lightly.
Tip out of the pan onto a plate and set aside.
Add the sprouts to the pan with a splash of water, then cover and finish cooking over a medium heat for about 5 minutes, stirring now and again, until just tender.
Uncover, turn up the heat, add the butter and cook the sprouts for 2 minutes more adding the pancetta and chestnuts, season with pepper and serve.