Preheat oven to 350 degrees. Line a 9x9 baking pan with parchment paper, spray with baking spray and set aside.
In a large glass mixing bowl combine the butter, oil, and chopped chocolate. Microwave in 30 second bursts, stirring after each round, until chocolate is fully melted and smooth (about 1 minute).
Whisk in sugars until smooth, thick, and glossy, about 2 minutes. Whisk in the eggs and vanilla until thick, about 1 minute more. Sift the flour, cocoa powder, salt, and espresso powder into bowl, then scatter 2 ounces of the dark chocolate chunks over top. Gently fold dry ingredients into batter, being sure to leave no streaks of flour behind.
Pour batter into prepared pan, top with remaining chocolate chunks, and bake in preheated oven for 18 - 20 minutes, or until a toothpick inserted into the center of the pan comes out with just a few crumbs on it.
Cool brownies completely to room temperature before slicing and serving.
Notes
Recipe adapted from Erin McDowell's book, The Fearless Baker.
Store leftover brownies wrapped in foil for up to a week.