Preheat the oven to 300 ̊. Set a rack in a large roasting pan and add enough water to come just below the rack. Place the pan over two burners on the stovetop and bring the water to a simmer over low heat.
Meanwhile, for the brisket: Combine the brown sugar, chile powder, garlic powder, paprika, salt, mustard powder and cayenne in a small bowl.
Trim most of the fat from the brisket, leaving about a 1/2-inch-thick layer. Rub the spice mixture all over the brisket. Turn off the heat under the roasting pan. Set the brisket fat-side up on the rack and cover the pan tightly with foil. Place the pan in the oven and bake until the brisket is very tender and the meat can be easily flaked with a fork but is not falling apart, 3 1/2 to 4 hours.
Meanwhile, for the mop: Combine the apple juice, cider vinegar, brown sugar, vegetable oil, Worcestershire sauce, salt and a few grinds of pepper in a small saucepan. Bring to a boil over medium-high heat and cook until the liquid is reduced to 1 cup, about 7 minutes. Remove from the heat and let cool.
When the brisket is finished roasting, preheat a grill to medium. Grill the brisket, brushing with the mop and turning occasionally, until the fat is crispy and the brisket is charred in spots, about 20 minutes. Remove from the grill and let rest 10 minutes.
Thinly slice the brisket against the grain and brush with the remaining mop. Butter the buns, then toast on the grill, about 1 minute. Serve the brisket with the buns, BBQ sauce, coleslaw and pickles.
