Preheat oven to 175°C (350°F). Line an 8x8 inch (20x20 cm) square baking pan with parchment, leaving an overhang for easy removal, and lightly grease the parchment with butter or nonstick spray.

Melt chocolate and butter

Place 170 g chopped bittersweet chocolate and 170 g unsalted butter in a heatproof bowl set over a gently simmering pot of water (double boiler), stirring occasionally until smooth and melted. Alternatively, microwave in 20–30 second bursts, stirring between, until fully melted. Remove from heat and let cool for 3–5 minutes.

Combine sugars and eggs

Whisk 300 g granulated sugar with the melted chocolate mixture until combined. Add 3 large eggs one at a time, whisking thoroughly after each addition to fully incorporate. Stir in 2 tsp vanilla extract.

Add dry ingredients

Sift together 95 g all-purpose flour and 30 g unsweetened cocoa powder directly into the chocolate-egg mixture to avoid lumps. Add 1 tsp fine sea salt and 1 tsp espresso powder (if using). Fold gently with a spatula until just combined — do not overmix. If using, fold in 60 g chocolate chips for pockets of melted chocolate.

Pour into pan

Scrape three-quarters of the batter into the prepared pan, spreading into an even layer with an offset spatula. Reserve the remaining batter for layering over the caramel later.

Salted Caramel

Make the caramel

In a medium heavy-bottomed saucepan, combine 200 g granulated sugar and 45 ml water over medium heat. Gently swirl the pan (do not stir with a utensil) until the sugar dissolves and begins to simmer. Increase heat to medium-high and cook without stirring until the mixture turns a deep amber color, about 8–10 minutes. Watch closely to avoid burning.

Finish the caramel

Remove the pan from heat and carefully whisk in 120 ml warm heavy cream in a slow, steady stream (mixture will bubble vigorously). Once the bubbling subsides, whisk in 30 g unsalted butter until smooth. Stir in 1 tsp coarse sea salt. Let the caramel cool for 5–8 minutes so it thickens slightly but is still pourable.

Assemble caramel layer

Pour the cooled-but-still-pourable salted caramel evenly over the brownie batter in the pan, leaving a small border. Sprinkle a few pinches of flaky sea salt across the caramel surface if you like a more pronounced salt finish.

Finish & Bake

Top with remaining batter

Drop spoonfuls of the reserved brownie batter over the caramel layer and use an offset spatula or the back of a spoon to gently spread/patch them to mostly cover the caramel. You want a marbled/patchy top so some caramel peeks through.

Bake

Bake in the preheated oven for 28–35 minutes, rotating the pan halfway through if your oven is uneven. The brownies are done when edges are set and a toothpick inserted into the center comes out with a few moist crumbs (not wet batter).

Cool and salt

Remove from oven and let the brownies cool in the pan on a wire rack for at least 1 hour to allow the caramel to set. While still slightly warm, sprinkle the surface with the remaining flaky sea salt (about 1 tsp). For cleaner slices, chill the pan for 30–60 minutes before slicing.

Serve

Lift the brownie slab from the pan using the parchment overhang. Slice into 9 or 12 squares using a sharp knife wiped clean between cuts. Serve at room temperature or slightly warmed; they pair well with vanilla ice cream.