Preheat oven to 350°F/180°C. Line an 8×8-inch baking pan with parchment paper or aluminium foil, leaving enough overhang on the sides for lifting the brownies out after baking.
In a large heat-proof bowl, place chocolate and butter. Heat in the microwave in 20-30 second intervals, stirring in between each interval, until melted (or set it over a saucepan of simmering water, stirring occasionally). Add sugar to chocolate mixture and whisk until combined. Add eggs and whisk the mixture vigorously for several seconds until shiny. Add flour and salt and fold, using a rubber spatula, just until combined.
Pour batter into prepared pan and smooth the top. Spoon dollops of caramel on top of brownie batter. Using a knife or skewer, gently swirl the two batters together to get a marble effect. Be careful not to over swirl and completely mix the batters tighter. You want to still see lots of the caramel blobs on top. If using, sprinkle coarse sea salt all over the surface of the batter.
Bake for 23-27 minutes, until the edges are set but the center is still very soft. Be careful not to overbake. A toothpick inserted into the center of the brownies should come out with moist crumbs, not dry. Let brownies cool completely on a wire rack, then refrigerate until set for easy cutting. Lift brownies out of the pan using the parchment paper and cut into squares. Serve cold or at room temperature.
Store brownies in the fridge in an airtight container for up to 4 days. Brownies can be frozen for up to 2 months. Thaw them overnight in the fridge.