To "Hasselback," place a potato on a cutting board. Snugly arrange 2 chopsticks or 2 identical round handles of wooden spoons at either side of the potato. (If needed, slice a slim piece from the bottom of the potato so that it will sit squarely.)
Using a sharp knife, make 1/8-inch slices along the length of the potato, using the chopsticks as a brake, so that while slicing, the blade stops before cutting all the way through the potato. Be aware of the ends of the potatoes, taking care not to slice all the way through. When finished, the top of the potato will fan out slightly. Place the potatoes in a bowl of ice water to keep them from browning while cutting the others.
In a medium bowl, stir together the softened butter, chives, parsley, thyme, sage, garlic, salt and pepper until well blended. Fold in the panko. (At this point, the butter may be shaped into a roll or stick and refrigerated for up to 1 week or frozen for up to 3 months.)
Preheat the oven to 400 degrees with the rack in the middle.
Thoroughly dry the potatoes. Using your fingertips, slather the potatoes with about half of the buttery crumbs, taking time to press the mixture between the slices. This will be challenging as the potato will be stiff and uncooperative. Once buttered, place the potatoes in a baking pan, casserole dish or cast-iron skillet, fitting them snugly in one layer. Place a piece of parchment over the potatoes and cover the dish with foil, sealing it well.
Bake for 30 minutes, remove the foil and parchment and plunge a fork into the center of the largest potato. It should yield and be soft but not collapse. If it is still hard, replace the parchment and foil and bake for an additional 15 minutes. If softened, draw the tines of the fork along the top of each potato to fan the slices. Plop nuggets of the remaining butter-crumb mixtures over the top of each potato.