How to Make It
If using a charcoal grill, open bottom vent of grill completely. Light charcoal chimney starter filled with briquettes. When briquettes are covered with gray ash, pour onto bottom grate of grill, and then push to one side of grill. Adjust vents as needed to maintain an internal temperature of 350°F to 400°F. If using a gas grill, preheat to medium (350°F to 400°F) on 1 side. If using an oven, preheat to 350°F.
Whisk together flour, salt, and baking soda in a small bowl; set aside. If grilling, place a heatproof bowl on unoiled grates over the side without the coals (or the unlit side of a gas grill). If using an oven, heat a medium saucepan over low. Add chocolate, butter, and oil to bowl or saucepan; cook, stirring constantly, until melted and smooth. Remove from heat. Let cool slightly, about 5 minutes. Add eggs, brown sugar, granulated sugar, and vanilla to chocolate-butter mixture; stir together until smooth and thoroughly incorporated. Add flour mixture; stir gently until just combined.
Grease a 10-inch cast-iron skillet with melted butter. Pour in batter, spreading in an even layer.
If grilling, place skillet on grates over the side without the coals (or the unlit side of a gas grill). Grill, covered, until a wooden pick inserted in center of brownies comes out almost clean (it will have crumbs but should not be wet), 40 to 45 minutes. If using an oven, bake in preheated oven about 35 minutes. Remove from heat; cool in skillet 10 minutes. Cut into wedges and serve with ice cream, if desired.