Preheat your oven to 175°C / 350°F. Line a 20 cm / 8 in square pan with parchment, leaving a little overhang so the brownies lift out easily later.
Put the butter and chopped dark chocolate in a heatproof bowl. Melt gently over barely simmering water, or microwave in short bursts, stirring often until smooth and glossy.
In a large bowl, whisk together the granulated sugar, brown sugar, and eggs for 2–3 minutes. The mixture should look slightly lighter and smoother; this helps create the shiny top.
Whisk in the vanilla. Slowly pour in the warm melted chocolate mixture and whisk until fully combined.
In a separate bowl, mix the flour, cocoa powder, salt, and espresso powder if using. This keeps the dry ingredients evenly distributed.
Fold the dry ingredients into the chocolate mixture using a spatula. Mix only until no dry streaks remain. Do not overmix, or the brownies can become cakier.
Fold in the chocolate chips or chunks for extra pockets of melted chocolate.
Pour the batter into the lined pan and smooth the top. Tap the pan lightly on the counter to release large air bubbles. Sprinkle lightly with flaky sea salt.
Bake for 28–32 minutes. The edges should look set, while the center should still be slightly soft. A toothpick inserted in the middle should come out with moist crumbs, not wet batter.
Let the brownies cool completely in the pan. Lift them out using the parchment, then slice with a warm clean knife for sharp, elegant squares.