Adjust oven rack to middle position and heat oven to 450 degrees.
Select and wash your potatoes, removing any dirt. Then dry off.
Spread 2 1/2 cups salt in even layer in 9 by 13-inch baking dish. Stir rosemary into the salt.
Nestle potatoes into the salt, leaving plenty space between potatoes so they are not touching.
Add the head of garlic, cut side up, to baking dish.
Cover baking dish with foil or a lid, covering tightly.
Bake 1 hour, 15 minutes, about 30 minutes in, remove garlic from pan so it doesn't burn. Then after the baking time, remove the pan from oven. And remove the foil or lid.
Increase oven temperature to 500 degrees.
Brush the potatoes with canola oil (1 tsp for each potato), and return to the baking dish, and the baking dish to the oven,
Bake another 15-20 minutes until potatoes are tender.
While potatoes finish baking, add garlic to melted butter to serve with potatoes, whisking it with a fork to evenly distribute garlic through the butter.
When potatoes are finished baking, brush any excess alt from the skin of the potatoes (carefully, they will be hot).
Use a fork to pierce potatoes in a cross shape, and squeeze potatoes to split them open to reveal the fluffy interior.
Serve immediately with the garlic butter or toppings of your choice.
Serve immediately.