Preheat the oven to 350°F (175°C). Grease and line a 9×13-inch baking pan with parchment paper.
In a large heatproof bowl, add dark chocolate and butter. Place the bowl over a pan of gently simmering water (double boiler method), making sure the bottom of the bowl doesn’t touch the water. Stir occasionally until the mixture is melted and smooth.
Remove the bowl from the heat. Whisk in the caster sugar and light brown sugar until fully combined.
Add the eggs and vanilla extract, mixing with a wooden spoon until well combined.
Sift the flour and sea salt over the mixture and stir until smooth.
Pour the batter into the prepared pan and smooth the top with a rubber spatula.
Bake in the preheated oven for 35-40 minutes, or until the edges are cracked and the center is soft. For extra chocolate, add chocolate chunks halfway through baking (at the 20-minute mark), then return to the oven to finish baking.
Allow the brownies to cool in the pan for about 10 minutes. Then transfer to a wire rack to cool completely before slicing into squares.