Preheat oven to 175°C (350°F). Line an 8x8 inch (20x20 cm) square baking pan with parchment paper, leaving an overhang on two opposite sides to lift the brownies out after baking. Lightly butter the exposed pan edges or spray with nonstick spray.

Melt chocolate and butter

Place the chopped chocolate and butter in a heatproof bowl set over a saucepan of simmering water (double boiler), making sure the bowl does not touch the water. Stir gently until fully melted and smooth. Alternatively, melt in 20-30 second bursts in the microwave, stirring between bursts to avoid scorching. Remove from heat and let cool for 3–5 minutes so it is warm but not hot.

Combine wet ingredients

Whisk the granulated sugar and eggs together in a medium bowl until combined. Add the vanilla extract and, if using, the espresso powder. Slowly pour the slightly cooled melted chocolate-butter mixture into the eggs and sugar while whisking constantly to temper the eggs and create a smooth, glossy batter. Continue whisking until fully homogeneous.

Incorporate dry ingredients

Sift together the flour, cocoa powder, and sea salt directly over the wet mixture (or whisk well to remove lumps). Fold the dry ingredients into the wet mixture using a rubber spatula or large spoon in gentle strokes until no streaks of flour remain. Avoid overmixing—fold just until combined.

Add mix-ins and transfer

Fold in the chocolate chips and chopped nuts if using, reserving a small handful to sprinkle on top. Scrape the batter into the prepared pan and smooth the top with an offset spatula. Lightly tap the pan on the counter to remove air bubbles. Sprinkle the reserved chips/nuts and the coarse sea salt evenly over the surface.

Bake

Bake in the preheated oven for 22–28 minutes. Check at 22 minutes: the edges should be set and slightly pulling away from the pan, while the center will still look slightly underbaked and glossy. For fudgier brownies, aim for the lower end of the time range; for a firmer but still moist brownie, bake a few minutes longer. Insert a toothpick into the center—if it comes out with a few moist crumbs (not wet batter), they are done.

Cool and serve

Remove the pan from the oven and place on a wire rack. Let the brownies cool completely in the pan (at least 1 hour) to set the fudgy interior. Use the parchment overhang to lift the brownies from the pan and transfer to a cutting board. Cut into 9 or 12 squares with a sharp knife, wiping the knife between cuts for clean edges. Serve at room temperature; warm slightly (10–12 seconds in microwave) if you prefer molten edges. Store leftovers airtight at room temperature up to 3 days, or refrigerate up to 7 days.