Melt 225g butter in a saucepan over medium heat, stirring constantly. Continue cooking until the butter turns golden and smells nutty, about 5 minutes. Pour into a large bowl and let cool for 10 minutes.
Whisk 200g brown sugar and 100g caster sugar into the cooled butter. Add 2 eggs and 1 tsp vanilla extract and whisk vigorously for 2 minutes until smooth and ribbon-like.
Fold in 280g plain flour, 1 tsp baking soda, and 1 tsp fine salt until just combined. Do not overmix.
Stir in 300g chocolate chips or roughly chopped dark chocolate. Cover the dough and refrigerate for at least 1 hour (overnight is even better).
Preheat oven to 190°C (375°F). Scoop dough into balls (about 60g each) onto lined baking trays, spaced 5cm apart. Bake for 11–13 minutes until edges are set but centres look underdone. Cool on the tray for 5 minutes before transferring.