Preheat oven to 350F with a rack in the middle position. Line a 9“ x 13” x 2“ anodized aluminum baking pan with foil (overhang to make it easy to remove brownies). Grease foil lightly with oil or pam
In sauce pan, melt butter over medium-low heat. When fully melted, increase to medium and simmer. Stir constantly, scraping the bottom, until silent and light folden brown. Remove from heat and stir in chocolate
In stand mixer with whisk attachment, combine sugars, salt, eggs, and vanilla. Whip on medium-high until extremely thick and fluffy (~8 min, but use visual cues as primary guide)
In a separate bowl, sift flour and cocoa powder
Reduce mixer speed to low, pour in chocolate-butter mixture
Pour cocoa-flour into mixer all at once and continue mixing until roughly combined. Finish by folding edges and bottom with a baking spatula
Pour mixture into foil-lined baking pan and bake until internal temperature is 205F (~30 min)
Cool completely and then slice into ~24 squares. Store in airtight container