Preheat oven to 350°. Butter a 9”-×-13” inch baking pan and line with parchment paper. In a medium bowl, whisk together flour, salt, and cocoa powder.
Set a large heat-proof bowl over a saucepan of simmering water to create a double-boiler. Add butter and 1 1/2 cups chocolate chips to bowl. Heat, stirring occasionally until the chocolate and butter are completely melted and smooth. Remove bowl from heat, then whisk in sugars and stir until combined.
Add eggs one at a time, stirring well between each addition, then stir in vanilla. Add flour mixture to chocolate mixture and fold in dry ingredients with a rubber spatula until just combined. Fold in remaining 1/2 cup chocolate chips.
Pour batter into prepared pan. Bake until a toothpick inserted into center of brownies comes out with a few moist crumbs, about 30 minutes. Let cool completely in pan.