Preheat your oven to 180°C/350°F (160°C fan-forced). Grease a 20 cm/8" square pan with butter and line it with baking paper.

Toast the walnuts on a tray for 5 minutes or until deeper in color, then let them cool.

Brown the butter: Place the butter in a silver saucepan over medium-high heat. Once melted, lower the heat to medium, allowing it to simmer gently. Keep stirring occasionally until it smells nutty and you see little golden specks. Pour the butter into a bowl immediately.

Add the sugar into the hot butter and mix to combine. Leave to cool for 3 minutes. Stir in the egg and vanilla until combined.

Add flour and salt, blending everything together until thick, resembling a flexible cookie dough. Finally, fold in the chocolate chips and walnuts.

Scrape the batter into your prepared pan, smoothing the surface with your hands. Bake for about 25 minutes, until the edges are lightly golden and a crackly layer appears on the surface.

Remove from the oven and let it rest in the pan for 10 minutes before transferring it to a cutting board. Cool for another 10 minutes then sprinkle with sea salt flakes before slicing into squares.