Taken from Napoleon’s Everday Gourmet Plank Grilling.
Preheat grill to medium-high heat. Pat bones dry with paper towel.
In a bowl, combine sea salt and pepper. Season both ends of the bones liberally with sea salt mixture.
Cut a 4-inch (10 cm) square of aluminum foil for each bone. Place largest end of each bone down on a square of aluminum foil; crimp the foil around the base of the bone to make a little foil tray to catch any drippings. Repeat with remaining bones.
Evenly space bones on plank. Place plank on grill and close lid. Plank grill the seasoned marrowbones for 20 to 30 minutes, until the bones are golden brown and about 25 percent of the marrow has melted and become liquid.
Remove bones from grill. Transfer to a serving platter. Serve with a small spoon or knife to scoop out the hot marrow and spread onto the bread.
NOTE: If you overcook these bones, you will end up with nothing to eat—because all the marrow will melt away. You want to melt only about 25 percent of the fat so that you get lots of hot, rich marrow to spread on the fresh bread.