Remove the rough exterior leaves of the Brussels sprouts along with any of the stem and discard. Remove a few layers of the greenish leaves until the yellow starts to show, keep the green leaves separate from the centers. Quarter the centers. Blanch the centers in salted boiling water until just al dente, about 4-5 minutes, and then spread on a large surface pan to cool. Very briefly plunge the green leaves into the water, remove and spread out to cool.
To finish the dish, use a fairly large non-reactive and not black-bottomed pan, place the butter in pan and put on a medium to medium-high heat. Cook the butter until golden brown and giving off a nutty aroma, add the grain mustard to the pan and toast slightly for just a moment. Add the Brussels sprouts and cook for about two minutes or just until the sprouts are hot throughout, add the vinegar and taste for seasoning. Add salt and pepper as necessary and finish with the parsley.