Preheat the oven to 350°F (175°C). Grease an 8x8-inch or 9x9-inch square baking pan and line with parchment paper, leaving an overhang on two sides for easy removal.

In a medium heat-proof bowl, combine the sliced butter and chopped bittersweet chocolate. Microwave in 30-second increments, stirring in between, until completely melted and smooth. Alternatively, use a double boiler over low heat.

In a large mixing bowl, combine the melted chocolate mixture with the granulated sugar and brown sugar. Whisk vigorously by hand for 1-2 minutes; the friction helps dissolve the sugar, which is essential for the crackly top.

Add the eggs one at a time, whisking well after each addition until the batter looks shiny and smooth. Stir in the vanilla extract.

Sift the cocoa powder, flour, and salt directly into the wet ingredients. Using a rubber spatula, gently fold the dry ingredients into the wet until just a few streaks of flour remain.

Fold in the semi-sweet chocolate chips until evenly distributed. Do not overmix, as this will incorporate too much air and create a cakey rather than fudgy texture.

Pour the batter into the prepared pan and spread it evenly with the spatula.

Bake for 30–35 minutes. Test for doneness by inserting a toothpick into the center; it should come out with several moist, fudgy crumbs attached (it should not be clean).

Remove from the oven and allow the brownies to cool completely in the pan for at least 1 hour. This allows the structure to set for clean slicing.

Lift the brownies out using the parchment overhang and cut into 16 squares using a sharp, non-serrated knife.