Prepare the Pan: Preheat the oven to 350°F (180°C). Prepare a metal 9×9-inch baking pan with non-stick spray or line it with parchment paper. Do not substitute an 8×8-inch pan, as the full recipe will not fit properly.
Combine Dry Ingredients: In a medium mixing bowl, whisk together the all-purpose flour, salt, and Dutch-processed cocoa powder until evenly combined.
Melt Chocolate Mixture: Place the chopped unsweetened chocolate, cubed unsalted butter, and 1½ cups of the granulated sugar in a small saucepan over low heat. Stir occasionally for about 5 minutes, continuing until the chocolate and butter are fully melted. Some of the sugar will dissolve, but the mixture should still appear grainy. Beat Sugar Eggs Chocolate: Add the remaining 1½ cups granulated sugar, eggs, egg yolks, and the melted chocolate mixture to a stand mixer or large mixing bowl. Beat for a full 3 minutes, until the mixture has doubled in volume. Set a timer and do not shorten this mixing period. Add Oil Vanilla: Add the vegetable oil and vanilla extract, then continue mixing on low speed until incorporated.
Incorporate Dry Mixture: Gradually add the flour, salt, and cocoa mixture while mixing on low speed. Continue only until the dry ingredients are fully combined. The finished batter will be thick.
Fill the Pan: Transfer the thick brownie batter to the prepared 9×9-inch metal pan and spread it into the pan.
Bake the Brownies: Bake at 350°F (180°C) for 38–45 minutes.
Let the Brownies Set: Remove the brownies from the oven and allow them to set for at least 4 hours before cutting. For the preferred texture, let them set overnight before slicing.
