Preheat the oven to 350 degrees F. (Or build your campfire and allow it to die down a bit, then move the embers to one side. Place a grate over top of the fire.) Lightly spray an 8-inch cast-iron skillet (with a lid if baking on a campfire) with baking spray.
In a large bowl (or a ziptop bag), combine the sugar, flour, chocolate chips, cocoa powder, espresso powder and salt. In a separate small bowl (or a jar with a lid), whisk (or shake) together the oil, vanilla and eggs. Add the wet ingredients to the dry and stir together (or smush if using a ziptop bag) until completely combined and incorporated.
Pour the batter into the skillet and spread in an even layer (if using a ziptop bag, cut the corner and squeeze into the skillet). Bake until the top of the brownie looks set and a toothpick inserted in the center comes out mostly clean with moist crumbs sticking to it, 25 to 30 minutes. (If baking on a campfire, top the skillet with a lid and bake for 30 minutes, then remove the lid, poke a few holes into a piece of tin foil and cover loosely. Bake for another 15 minutes.)