Preheat the oven to 175°C (350°F). Line an 8-inch (20 cm) square baking pan with parchment paper, leaving a 2-inch overhang on two opposite sides for easy lifting. Lightly grease the parchment with butter or nonstick spray.

Melt chocolate and butter

Combine the chopped chocolate and butter in a heatproof bowl. Melt gently using a double boiler (place bowl over simmering water, making sure water doesn't touch the bowl) or in 20–30 second bursts in the microwave, stirring between bursts until smooth. Remove from heat and let cool for 3–5 minutes.

Combine sugar and eggs

In a large mixing bowl, whisk the granulated sugar, eggs, and vanilla together until glossy and slightly thickened (about 1–2 minutes by hand or 30–45 seconds with a hand mixer on low). This helps incorporate air for a slightly lighter crumb.

Incorporate chocolate

Slowly pour the melted chocolate-butter mixture into the sugar-egg mixture while stirring continuously to combine. Stir until uniform and slightly glossy.

Add dry ingredients

Sift the flour, cocoa powder, and fine sea salt (and espresso powder if using) together over the bowl. Fold gently with a rubber spatula just until no streaks of flour remain. Avoid overmixing to keep brownies fudgy.

Fold in add-ins

Fold in chopped nuts and/or chocolate chips if using, reserving a small handful to sprinkle on top.

Transfer to pan

Scrape the batter into the prepared pan and smooth the top with an offset spatula. Sprinkle remaining chips/nuts and a light pinch of coarse sea salt over the top for contrast.

Baking and Finishing

Bake

Place the pan in the preheated oven on the middle rack. Bake for 22–28 minutes. Check at 22 minutes: the edges should be set and the center should still look slightly underbaked and glossy; a toothpick inserted into the center should come out with a few moist crumbs (not wet batter) for fudgy brownies. For cakier brownies, bake to 30–35 minutes until a few more crumbs cling.

Cool

Remove the pan from the oven and place on a wire rack. Let cool in the pan for at least 1 hour to set. If you want warm squares, cool 30 minutes. Use the parchment overhang to lift the brownies from the pan and transfer to the rack to cool completely before slicing for clean edges.

Slice and store

Using a sharp knife wiped clean between cuts (warm water and dry), slice into 9 or 12 squares depending on desired size. Store in an airtight container at room temperature up to 3 days, or refrigerate up to 7 days. For longer storage, freeze tightly wrapped for up to 3 months; thaw at room temperature.