Preheat the oven to 325℉. Grease and line an 8x8-inch pan with parchment with overhang.
Combine flour, ½ cup chocolate chips, cocoa powder, and salt in a small bowl; set aside.
Melt 6 ounces chocolate chips and butter in a large stainless steel bowl over simmering water; stir occasionally until smooth. Remove from heat and whisk in sugars until well combined. Set aside.
Whisk together eggs and vanilla in a small bowl; whisk into chocolate mixture until incorporated. Whisk in flour mixture until just combined. Transfer batter to the prepared pan.
Bake in the preheated oven until an inserted toothpick comes out with wet crumbs, about 25 minutes. Allow to cool for 1 hour at room temperature, then chill completely in the refrigerator before slicing.
Store brownies in an airtight container at room temperature, in the refrigerator, or in the freezer for longer storage.