Preheat the oven to 180°C (350°F). Line an 8-inch square baking pan with parchment paper, leaving an overhang for easy removal.

Cook 100 g unsalted butter in a heavy-bottom saucepan over medium heat. Continue cooking until the milk solids turn golden brown and smell nutty.

Remove the browned butter from the heat. Set it aside to cool slightly.

Melt 200 g dark chocolate in a heatproof bowl over a double boiler, or microwave it in short bursts. Stir until smooth.

Let the melted dark chocolate cool slightly. Keep it fluid but not hot before combining it with the eggs.

Crack 3 eggs into a large mixing bowl. Add 120 g caster sugar and 120 g brown sugar.

Whisk the egg and sugar mixture until it becomes light and airy. Whisk in 30 ml espresso until evenly combined.

Pour the slightly cooled melted dark chocolate into the egg mixture. Add the browned butter.

Whisk the batter until the chocolate and browned butter are fully incorporated. Stop mixing once the batter looks glossy and uniform.

Sift 60 g all-purpose flour and 30 g cocoa powder over the batter. Fold gently until no dry streaks remain.

Transfer the brownie batter to the lined 8-inch square baking pan. Spread the surface into an even layer.

Bake at 180°C (350°F) for 25 to 30 minutes. Remove when the edges are set but the center is still slightly wobbly.

Transfer the pan to a wire rack. Cool the brownies completely before removing them from the pan.

Lift out the cooled brownie slab using the parchment paper. Slice into 9 squares and serve, optionally warm with vanilla ice cream.