Preheat oven to 175° C (350 degrees F). Line a 23 x 23 cm (9x9-inch) baking pan with parchment paper or aluminum foil, leaving an overhang on the sides to lift the finished brownies out (makes cutting easier!). Set aside.
In a medium saucepan on medium heat, combine the butter, oil, light brown sugar, and espresso powder (if using). Stirring frequently, until butter is completely melted. You want this mixture to be HOT!
In a large mixing bowl (or in the bowl of your standing mixer), combine the eggs and granulated sugar. Whisk until lighter in color. ( 5-6 mins)
Slowly, pour the butter mixture into the egg mixture, carefully, adding it a little bit at a time and whisking constantly until completely combined. This won't take too long.
Add in the flour, salt, cocoa powder, and chocolate chips. Use a rubber spatula to gently fold the dry ingredients into the wet ingredients until JUST combined (do NOT over beat as doing so will affect the texture of your brownies).
Pour batter into prepared pan, smoothing the top out evenly with spatula! (Important)
Bake for 27 to 32 minutes, or until the edges are firm and the top is shiny and slightly cracked.
Brownies will continue baking and set in the hot pan out of the oven. If there are a few moist crumbs on the toothpick, the brownies are done. You don’t want a clean toothpick!
After 15-20 minutes, carefully remove them out of the pan and allow to cool to room temperature before slicing into brownies. They set while they cool.