Grease and line a 9 x 13″ baking pan with parchment paper. Butter parchment paper well.
Add 1 1/2 cups Butter, and 1 1/2 cups Semisweet Chocolate Chips to a small saucepan and cook over medium-low heat, stirring until chocolate melts.
Remove from heat and set aside.
Stir in 1 cup Brown Sugar, 1 cup Granulated Sugar, and 1 teaspoon Salt, until sugar is dissolved.
Allow mixture to rest for 5-10 minutes, until close to room temperature, so eggs don’t curdle.
Whisk in 6 whole Eggs, 1 at a time beating well between each addition. Stir in 1 teaspoon Vanilla, until well combined.
In a small bowl whisk together 2/3 cup Dark Chocolate Cocoa Powder, and 3/4 cup All-Purpose Flour or Whole Wheat Pastry Flour, until well combined.
Whisk flour mixture into melted chocolate mixture.