Heat the oven to 350°F (175°C). Line an 8-inch square baking pan with parchment or nonstick foil so it overhangs the sides and lightly grease the liner; set the pan aside.

Melt 10 ounces of dark chocolate using a double boiler or in the microwave. If using the microwave, warm in 30-second bursts at about 70% power, stirring between bursts until smooth. Allow the chocolate to cool briefly.

In a large bowl, cream the room-temperature butter and granulated sugar with a handheld mixer on medium-high speed until the mixture becomes smooth and creamy, about 3 minutes.

Add the eggs one at a time, beating well after each addition so the batter becomes cohesive and glossy.

Stir in the vanilla extract and kosher salt, then beat on high for roughly a minute until the mixture lightens slightly and gains air.

Sift or sprinkle in the unsweetened cocoa powder and fold it in by hand to start, then mix briefly on low speed until the powder is fully incorporated.

Pour the cooled melted chocolate into the batter and beat until the mixture is smooth and glossy, about 2 to 3 minutes.

Spoon the batter into the prepared pan, spread it into an even layer, and firmly tap the pan on the countertop to remove large air bubbles.

Evenly scatter the optional chocolate chips over the surface and press them lightly so they stick into the batter.

Bake on the center rack for 22 to 25 minutes, until the top has puffed and set and the center no longer jiggles when you gently shake the pan—be careful not to overbake.

Remove the pan from the oven and let it cool completely on a wire rack while remaining in the pan.

When the brownies are mostly cool, refrigerate the pan for about 1 hour until the brownies feel firm; this will produce cleaner slices. Lift the brownies from the pan using the parchment overhang and cut into 9 squares with a sharp knife.