1 Preheat the oven to 350°F. Lightly grease an 8x8-inch light-colored metal baking pan and line it with parchment paper, leaving an overhang on two sides.
2 Add the butter to a medium saucepan over medium heat. Cook, stirring frequently, as the butter melts, foams, and begins to turn golden. Continue cooking until the milk solids at the bottom are deep golden brown and the butter smells nutty.
3 Immediately remove the pan from the heat. Add the chopped bittersweet chocolate and let it sit for about 30 seconds. Whisk until completely melted and smooth.
4 Whisk the oil and cocoa powder into the warm chocolate mixture. The heat helps bloom the cocoa and intensify its chocolate flavor. Set aside to cool until warm, but no longer hot.
5 Meanwhile, add the eggs, egg yolks, granulated sugar, and brown sugar to a large mixing bowl. Using an electric hand mixer, beat on medium-high speed for 3–4 minutes, until the mixture is noticeably lighter, thicker, and glossy.
6 Add the powdered sugar, vanilla, and salt. Beat for another 1–2 minutes, until smooth and glossy.
7 Slowly pour the warm chocolate-brown-butter mixture into the egg mixture while whisking on low speed. Mix just until smooth and completely incorporated.
8 Add the flour and gently fold with a silicone spatula until only a few streaks of flour remain.
9 Fold in the chocolate chips or chopped chocolate until just combined. Don't continue mixing once the flour disappears.
10 Pour the batter into the prepared pan and spread it evenly into the corners. Sprinkle a few additional chocolate chunks over the top if desired.
11 Bake for 28–34 minutes. The edges should look set and slightly puffed, the top should be shiny and crackled, and the center should no longer look liquid but may have the slightest wobble when you gently move the pan.
12 A toothpick inserted about 1 inch from the edge should have moist crumbs. One inserted into the center can have thick, fudgy crumbs and melted chocolate, but shouldn't be coated in completely raw batter.
13 Immediately after removing the brownies from the oven, gently tap the pan once or twice against the counter. This helps settle the center into that dense, fudgy texture.
14 Sprinkle with flaky sea salt, if desired. Let the brownies cool completely in the pan for at least 1½–2 hours before lifting them out and slicing.
15 For super clean, dense slices, refrigerate the cooled brownies for about 30 minutes before cutting.
