In a large pot of salted boiling water, cook potatoes until tender, about 15 to 20 minutes. Place in a colander and drain well. Set aside for 10 to 15 minutes to allow excess water to evaporate.
Meanwhile, combine the cream and butter in the pot and warm slightly.
Return the potatoes to the pot and mash together with the butter-cream mixture, seasoning with the chopped herbs and salt and pepper. Take care not to over-mash — a few lumps are desirable.
Set aside to cool to room temperature, cover, and refrigerate until needed. (These steps can be done the day before serving.)
Preheat grill to medium-high 450ºF.
Form the chilled potatoes into uniform, level, mounds on the pre-soaked cedar planks.
Roast over direct heat for 5 to 10 minutes, or until the potatoes are golden brown and have formed a crisp crust.
Carefully remove the smoking plank from the barbecue and move potatoes to a heatproof platter, or directly to plates to serve immediately. Garnish with extra chives.