Preheat oven to 350°F (175°C). Line an 8-inch square baking pan with parchment paper leaving a 1–2 inch overhang for easy removal; lightly grease the paper and exposed pan edges with butter or nonstick spray.

Measure and have all ingredients at hand. If you choose to use espresso powder, mix it with the cocoa powder so it dissolves evenly into the batter.

Make the Brownie Batter

Melt chocolate and butter

In a heatproof bowl set over (not touching) simmering water (double boiler), combine the bittersweet chocolate and butter. Stir occasionally until smooth and fully melted, about 4–6 minutes. Alternatively, microwave in 20-second bursts, stirring between each, until smooth. Remove from heat and let cool 3–5 minutes.

Combine sugars and eggs

Whisk the granulated sugar and brown sugar into the slightly cooled chocolate-butter mixture until incorporated. Add the eggs one at a time, whisking thoroughly after each addition so the mixture becomes glossy. Whisk in the vanilla extract.

Add dry ingredients

Sift together the all-purpose flour, unsweetened cocoa powder (mixed with espresso powder if using), and sea salt over the chocolate mixture. Fold gently with a rubber spatula just until no streaks of flour remain — do not overmix to keep the brownies fudgy.

Add mix-ins

Fold in half (about 1/2 cup) of the dark chocolate chunks and half of the chopped toasted walnuts into the batter, reserving the rest for the top.

Bake & Finish

Transfer to pan

Scrape the batter into the prepared pan, smoothing the top with an offset spatula or the back of a spoon to an even thickness (about 1 inch). Scatter the remaining chocolate chunks and walnuts evenly over the top. Lightly sprinkle the coarse sea salt over the surface.

Bake

Bake in the preheated oven for 20–26 minutes, rotating the pan halfway through. The brownies are done when the edges are set and a toothpick inserted into the center comes out with a few moist crumbs (not wet batter). Baking time will vary with oven and pan thickness.

Cool & slice

Remove pan from the oven and let cool on a wire rack for at least 30 minutes. Use the parchment overhang to lift brownies from the pan and transfer to a cutting board. For cleaner slices, chill for 30 minutes, then cut into 9 squares with a sharp knife, wiping the blade between cuts.

Serve at room temperature. Store leftovers in an airtight container at room temperature for up to 3 days or refrigerated for up to 7 days. Rewarm slightly before serving if chilled.