- Set the heat to medium-low and immediately whisk in the butter, followed by the cream, until smooth and glossy. Be careful here! The mixture will bubble and spurt as you add the dairy, but will subside as it continues to cook.
- Pre-heat the oven to 180°C. Grease and line a 20cm square baking pan with non-stick baking paper. Let the paper slightly overhang on the sides - it'll help you lift the brownie out of the pan later.
- Put the butter and sugar into a medium-sized saucepan. Heat over low-medium heat and whisk often until the sugar is dissolved and the mixture is shiny and smooth. If you have a candy thermometer, you can tell it's ready when it registers at 50°C.
- Meanwhile, in the bowl of a stand mixer fitted with the whisk attachment or using handheld electric beaters, whisk together the eggs, cocoa powder, espresso or coffee powder (if using) and salt on medium speed until the mixture is glossy and flows like lava when the whisk is raised — for about 2 minutes.
- When the melted butter mixture is ready, slowly pour it into the egg mixture, whisking continuously on medium speed until combined. The mixture should be very thin and shiny. Pause to scrape down the bottom and sides of the bowl. Sift over the flour and whisk until just combined.
- Remove the bowl from the stand mixer and use a large wooden spoon or rubber spatula to fold in the dark chocolate chunks until evenly incorporated throughout the batter. The batter should be warm enough to partially melt some of the chocolate, but not all of it, resulting in pockets of melting chocolate in the baked brownie.
- Pour the mixture into the prepared baking pan and use an offset palette knife or the back of a metal spoon to evenly smooth out the top. Drizzle the caramel across the top of the brownie in swirled motions. Use a clean butter knife or wooden skewer to drag the caramel through the batter to create a swirl-like effect. Sprinkle a small pinch of flaked salt over the top of the brownie, if desired.
- Bake for 30 minutes or until firm around the edges and just set in the middle. A skewer inserted into the middle should not come out clean but with a few moist crumbs attached. Remove from the oven and allow the brownie to rest in its pan until completely cool in temperature.
- When you're ready to serve, remove the brownie from the pan and use a sharp knife to slice it into even squares of desired size. These brownies are best eaten on the day of making, but can be kept in an airtight container stored in the refrigerator for up to five days.