Preheat oven to 350 degrees Fahrenheit. Grease a 9x13 baking dish with coconut oil or butter and set aside.
In a microwave safe bowl, heat the butter, bittersweet, and unsweetened baking chocolate at 50% power for 30 second intervals, stirring in between, until the chocolate is all melted. Set aside.
In the bowl of a stand mixer, combine the eggs, sugars, and vanilla extract. Beat at medium speed for 3-5 minutes, or until lifting the paddle from the bowl produces ribbons.
On low speed, beat in the slightly cooled melted chocolate.
Add the salt and flour, mixing only until just combined.
Remove bowl from mixer and gently fold in the chocolate chips and nuts.
Spread into the prepared baking dish and bake for 40-45 minutes, or until a toothpick inserted in the center comes out slightly moist.
Cool for at least a half hour (preferably completely) before slicing and serving.