Tie the pork butt with twine in several places so that it cooks evenly. Salt it generously all over and let it sit at room temperature for about an hour.
In the meantime, combine the maple syrup, brown sugar, mustard, thyme, garlic and chipotle powder in a small bowl. Add a few pinches of salt and several grinds of pepper.
Heat the oven to 475° F. When the pork is at room temperature and the oven is hot, smear the sugar, mustard and garlic mixture all over the pork, concentrating a good amount of it on the top of the roast, where the fat is. Nestle the pork (fat-side-up) into a roasting pan or cast iron baking dish just big enough to hold it, and put it in the oven. When you start to smell garlic and sugar burning, and after no longer than 15 minutes, turn the heat down to 200 degrees. (Do not open the oven door to peek!)
Leave the pork in the oven overnight, for at least 6 hours and up to 8. When you wake up in the morning your house will smell intoxicating, and the pork will be ready to shred and pack up for lunches for the whole family -- all you need is a soft roll and some coleslaw or pickled fennel, or a big pile of mashed potatoes.