Nonstick vegetable oil spray or unsalted butter, room temperature
60g all-purpose flour
50g Dutch-process cocoa powder
3/4tsp. Morton kosher salt
115g unsalted butter
115g dark chocolate, preferably 70%, broken into pieces
250g granulated sugar
3 large eggs
1 tbsp. vanilla extract
115g milk chocolate chips
Preheat oven to 375F/190C/170C fan. Lightly coat a metal 8×8″ baking pan with nonstick vegetable oil spray or unsalted butter, room temperature, then line with parchment paper, leaving generous overhang on 2 sides. Lightly coat parchment with nonstick spray.
Whisk all-purpose flour, cocoa powder and kosher salt in a small bowl to combine.
Combine unsalted butter and dark chocolate in a medium metal bowl set over a large saucepan of barely simmering water (bottom of bowl should not be touching water). Heat, stirring occasionally, until chocolate is melted and smooth, about 5 minutes.
Whisk in granulated sugar, then add 3 large eggs one at a time, whisking very vigorously between additions. Once all eggs are incorporated, continue to whisk 1 minute more. Remove bowl from heat and whisk in vanilla extract, then whisk in dry ingredients. Using a rubber spatula, stir in milk chocolate chips. Scrape batter into prepared pan; smooth surface.
Bake brownies until set across top and a tester inserted into the center comes out with just a few moist crumbs attached, 27–30 minutes. Let cool in pan at least 30 minutes, preferably in refrigerator, before cutting into 12 pieces.