Slice the two bok choy in half then cut out the firm core leaving enough behind to hold the leaves together. Wash any grit or dirt caught between the leaves.
Cook the bok choy cut side down on the grill until it starts to lightly brown, about 5 minutes. Flip the bok choy.
Add the rub and butter to the now cut-side up. The butter will slowly melt and pool in the leaves.
When the bok choy has started to brown on the other side, the butter fully melted, and the leaves softened the bok choy is ready to serve.