1. Line a 9×13 pan with aluminum foil or parchment paper and spray with cooking spray.
2. Combine chocolate and butter and set over a pan of simmering water.
3. Heat, while occasionally stirring, until butter and chocolate are melted and smooth.
4. Remove from heat.
5. Whisk in cocoa powder.
6. Whisk together the eggs, sugar, salt, and vanilla in a separate bowl.
7. Add the chocolate mixture and mix well. 
8. Fold in the flour with a spatula until incorporated and no dry streaks remain. They are excellent without any additions, but if you want to add walnuts or extra chocolate chips for texture, do so now and gently fold them in.
9. Spread into the prepared pan in an even layer, bake at 350F, and set time for 35 minutes.
10. Remove from oven and leave to cool completely.

Enjoy!