Preheat your oven to 175°C (350°F). Line an 8x8-inch (20x20 cm) baking pan with parchment paper, leaving some overhang for easy lifting.

Combine the melted butter and chocolate in a large, heatproof bowl. You can microwave them in 20-second bursts, stirring between them, until fully melted and smooth, or you can melt them using a double boiler. Let cool slightly.

Whisk in sugar and brown sugar to the slightly cooked melted butter and chocolate. Add eggs one at a time, mixing vigorously after each to create a shiny, thick batter. It will look like a foamy peanut butter consistency.

Stir in vanilla extract, espresso powder (if using), salt, and cocoa powder. Add flour and mix gently just until combined — don’t overmix.

Fold in chocolate chips. Save some to add to the top before baking.

Pour the batter into the prepared pan and spread evenly. Bake for 20-25 minutes until the edges are set and a toothpick inserted in the center comes out with moist crumbs (not raw batter).

Sprinkle with flaky sea salt.

Let brownies cool in the pan for at least 30 minutes before lifting and slicing. For clean edges, chill in the fridge for 15–20 minutes before cutting.