Preheat the oven to 180°C. Grease a 23x23cm baking pan/tin and line the inside with parchment paper, creating a slight overhang so you can lift the brownies out once baked.

In a medium bowl, combine the butter and chocolate. Place the bowl over a pot of gently simmering water. Stir the mixture occasionally until it has completely melted. Remove the bowl from the pot and allow the mixture to cool for 10 mins. It’s really important to let it cool to prevent it from scrambling the eggs when you add them.

In the large bowl of a stand mixer with the whisk attached, combine the eggs, egg yolks, granulated/caster sugar, and light brown sugar. Whisk for 3 to 4 mins or until the mixture becomes pale and fluffy. This is the secret to getting that nice shiny top on your brownies! (You can also mix these ingredients with an electric hand mixer.)

Once it is thick, very very slowly drizzle in the cooled melted chocolate mixture while still whisking on a low speed. It's important to do this slowly so the eggs can slowly adjust to the temperature of the warm chocolate.

Once the chocolate has completely incorporated, sieve in the flour, cocoa powder and salt. Use a hand whisk to gently mix these in until no more dry ingredients are visible. Don’t over mix.

Pour the batter into the pan/tin, spreading it to the edges. Sprinkle the chocolate chunks and a pinch of sea salt flakes over the top.

Place the pan/tin in the oven and bake for 24 or 27 mins. A wooden skewer inserted in the middle should still come out slightly wet.

Remove the pan/tin from the oven and allow the brownies to cool to room temperature. Refrigerate for 30 mins.

Carefully lift the cooled brownies from the tray and use a hot knife to cut them into slices before serving.