Preheat the oven to 325*F with a rack in the center. Grease a 9” square pan and line with parchment.
Melt the butter in a medium saucepan over medium heat and then stir in the light brown sugar and 2 tablespoons of water until combined. Bring to a simmer and then reduce the heat to low and simmer for 5 minutes.
Pour into a large heatproof bowl and then whisk in the oil. Set aside for 5 minutes to cool.
Stir in the cocoa powder and the salt, the mixture may look grainy and broken- that is okay.
Whisk in the eggs, yolk and vanilla until the mixture is smooth, about one minute, then use a spatula to stir in the flour and chocolate chips.
Pour into the prepared pan and smooth the top.
Bake for 36-40 minutes, or until a toothpick inserted in the center comes out with moist crumbs.
Cool completely before removing from the pan and slicing.
