In a bowl, whisk together the milk powder, Parmesan, chives, dill, onion powder, granulated garlic, parsley, salt and sugar. Mix thoroughly to combine.
Place the olive oil in a separate bowl and mix in 1 tablespoon of the spice mixture.
Insert a bamboo skewer lengthwise through the center of each potato. With a sharp paring knife, cut a potato in a spiral pattern all the way down to the skewer, beginning at the top of the potato with the knife set at a 15 to 20 degree angle. Rotate the potato as you cut until the entire the potato is cut through in a spiral pattern. Stretch the potato down the skewer. Repeat with the remaining potatoes.
Set each skewer over a piece of aluminum foil. Brush each of the potatoes with a generous amount of the oil mixture, reserving a third for later. Wrap the potatoes in the foil, closing both ends so the oil cannot escape. Place on the grill and cook 8 minutes, then flip and cook 8 minutes more. Remove from the grill and let rest until cool enough to handle. (Leave the grill on.)
Remove the skewers from the foil. Brush each potato with some of the remaining oil mixture and return to the grill until well browned, 2 to 3 minutes per side. Transfer to a platter and sprinkle each with more of the dry spice mixture. (Extra spice mixture can be stored in an airtight container at room temperature for up to 2 months.)