Preheat oven to 175°C (350°F). Lightly butter an 8x8-inch (20x20 cm) square baking pan, then line it with parchment paper leaving an overhang on two opposite sides for easy removal. Butter the parchment or spray lightly with nonstick spray.

Melt chocolate and butter

Place the chopped bittersweet chocolate and 170 g butter in a heatproof bowl. Set the bowl over a gently simmering saucepan of water (double boiler), ensuring the bowl doesn't touch the water. Stir occasionally until smooth and fully melted. Alternatively, microwave in 20–30 second bursts, stirring between bursts, until melted and glossy. Remove from heat and let cool 3–5 minutes.

Combine sugars and eggs

In a medium bowl, whisk together the granulated sugar and light brown sugar. Add the eggs one at a time, whisking or using a handheld mixer on low until fully incorporated and the mixture is glossy. Stir in the vanilla extract.

Temper chocolate into eggs

Slowly pour about one-quarter of the warm melted chocolate-butter mixture into the egg-sugar mixture while whisking continuously to temper the eggs. This prevents curdling. Then pour the tempered egg mixture back into the remaining chocolate and whisk until smooth and uniform.

Add dry ingredients

Sift (or whisk) the all-purpose flour, cocoa powder, fine sea salt, and espresso powder together. Fold the dry ingredients into the chocolate mixture with a rubber spatula in two additions, mixing gently until no dry streaks remain. Do not overmix—stop when just combined to keep the brownies fudgy.

Add-ins & Baking

Fold in add-ins

If using chocolate chips and/or toasted nuts, fold them into the batter now, reserving a small handful to scatter on top for appearance.

Fill pan and smooth

Spoon the batter into the prepared 8x8 pan and smooth the top with an offset spatula. Tap the pan gently on the counter 2–3 times to release any large air bubbles.

Bake

Bake in the preheated oven for 22–28 minutes. Test for doneness by inserting a toothpick near the center: for fudgy brownies, it should come out with a few moist crumbs attached (not wet batter). Oven times vary—start checking at 20 minutes.

Cool and finish

Remove pan from oven and immediately sprinkle reserved chocolate chips and the optional flaked sea salt over the hot surface so they melt slightly and adhere. Cool the brownies in the pan on a wire rack for at least 1 hour to set (longer if you want cleaner slices). Use the parchment overhang to lift the brownie slab from the pan, then cut into squares with a sharp knife. For neat slices, chill 30 minutes before cutting.

Serving

Serve the brownies at room temperature or slightly warmed. Optionally top with a scoop of vanilla ice cream or a dusting of cocoa powder. Store airtight at room temperature up to 3 days, or refrigerated up to 7 days. For longer storage, freeze fully cooled brownies wrapped tightly for up to 3 months.

Notes

For extra fudginess, slightly underbake and let set completely as they cool. Using higher cocoa content chocolate and minimal flour yields the richest, fudgiest texture. If you prefer cakier brownies, increase flour to 120 g and bake a few minutes longer.