My family tells me they're the best mashed potatoes ever and I think I'd agree.  The cedar adds a wonderful light smoky flavor to them as well as making a very light golden crust on the outside.

This is based off a recipe from Ted Reader's "King of the Q's Blue Plate BBQ" book.  The potatoes are started the day before by boiling them with garlic and onions.

Then they're mashed and mixed with half and half, ranch dressing, spices, herbs, cheddar cheese, and string cheese and put in the refrigerator for 24 hours.

When ready to grill, take them out and form into a nicely pressed and smoothed out mound on the cedar plank. 

Grill at about 400 degrees for about 30 minutes to let the cedar flavor set in and develop the light crust.

This picture didn't turn out too well, but you get the idea.  The shredded cheddar just melts into the potatoes but the chunks of string cheese stay as nicely melted chunks in the potatoes.