Preheat your oven to 180°C (350°F). Grease your 9x13 inch baking pan and line it with parchment paper, leaving some overhang for easy removal.
Melt the butter in a microwave-safe bowl in short bursts, stirring until smooth and shiny, then let it cool slightly to avoid cooking the eggs.
In a large bowl, whisk together the cocoa powder, sugar, and salt until evenly combined. Pour in the cooled melted butter and stir until the mixture is dark, glossy, and fragrant.
Add the eggs one at a time, whisking well after each addition until the batter becomes thick and shiny. Stir in the vanilla extract for a warm aroma.
Sift the flour into the batter and gently fold it in with a spatula, just until no streaks remain. Be careful not to overmix to keep the bars tender.
Chop the dark chocolate or chocolate chips and fold them into the batter, distributing pockets of gooey chocolate throughout.
Pour the batter into your prepared pan and spread it evenly with a spatula, smoothing the top for a level bake.
Bake in the oven for 25-30 minutes, until the edges are crackly and set, and a toothpick inserted in the center comes out with moist crumbs.
Remove the pan from the oven and let the bars cool in the pan for about 20 minutes. Then, lift them out using the parchment overhang and transfer to a wire rack to cool completely.
Once cooled, use a sharp knife to cut into bars, wiping the blade between slices for clean edges. Serve slightly warm or at room temperature for maximum gooeyness.
