Cut six to eight 12X9 inch rectangles of heavy-duty aluminum foil.
Make 4 or more slices, side by side, into the potatoes, taking care NOT to cut the whole way, just about 3/4 of the way down. (an easy way to do this is to put the potato on a large serving-type spoon, and begin to slice downward. Your knife will stop when it hits the edges of the spoon.)
Fan-open the potatoes, gently.
Place 2 on each piece of foil.
Drizzle the butter over top of potatoes and sprinkle with salt, pepper, chives and tarragon to taste. Seal the foil to form packets.
Place in oven and cook until tender, approximately 1 hour.