Either grease a 13x9 inch pan with cooking spray, or line it with parchment paper, and preheat oven to 335 degrees F.
In a microwave-safe bowl, combine dark chocolate, semi-sweet chocolate, and butter. Microwave on high in 30 second intervals, stirring between each one until completely melted and blended. Set aside to cool.
In a mixing bowl, whisk together flour, cocoa powder, and salt. Set aside.
In the bowl of a stand mixer, combine sugar, eggs, and vanilla extract. Beat about 3 minutes, scraping the sides of the bowl frequently. Add cooled chocolate/butter mixture, and beat for another 3 minutes.
Gently fold in dry ingredients and chocolate chips.
Spread in prepared 13 x 9 pan, and bake at 335 degrees for 20-25 minutes. Do not over-bake.
Cool on wire rack.
Fun Variations: Add 1 cup chopped walnuts, Andes mint pieces, peanut butter chips, toffee bits, crushed peppermints, or other baking morsels at the same time as the chocolate chips and dry ingredients.