Prepare a medium-hot fire in a charcoal or gas grill. Put bok choy stalks in a large bowl. Using your hands (or a fork), coat bok choy with miso butter. Arrange bok choy, cut side down, on grill grate. (If you have a grill screen, set it on top of the grate before adding the bok choy, to keep the stalks from falling through the gaps.) Close lid and grill for about 5 minutes, until golden brown on the underside. Turn bok choy with tongs, re-cover, and grill for 5–6 minutes more, until golden and crisp-tender.