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