First make the brown butter: pop the diced butter into a medium-sized non-stick saucepan and set over medium heat. Stir gently until completely melted. Increase the heat to medium and allow the butter to bubble, stirring often. It will bubble and spit - don’t panic, this is normal (just take care!). Continue to cook the butter, stirring to ensure it cooks evenly, until the crackling noises fade and the bubbles subside. At this point, you should see a foam appear on the surface and the colour will change from golden-yellow to light brown to deep amber brown. This can take anywhere between 7-10 minutes, depending on the size of your pan and how high your heat is. Watch the pan closely as it takes just seconds to go from brown to burnt.
As soon as brown flecks start to appear and you start to smell a gorgeous nutty aroma, remove from the heat. Quickly, but carefully, pour into a medium-sized heatproof bowl. This stops it cooking any further. Leave to cool slightly. It doesn’t need to be room temperature but shouldn’t be scorching hot. You can make the brown butter up to 2 weeks in advance and keep it chilled in your fridge. Allow to come to a softened room temperature before making the cookies.
In a large mixing bowl, add the brown butter, caster sugar and soft light brown sugar. Using an electric whisk, beat together. You can use a wooden spoon too.
Add the egg, egg yolk and vanilla extract and mix again until combined.
In a separate large mixing bowl, sift together the plain flour, bicarbonate of soda and salt. Mix half the dry ingredients into the wet ingredients until combined. Gradually mix in the remaining dry ingredients, a little at a time, until you have a smooth dough.
Gently fold in the chopped chocolate chunks. Take care not to overmix, just carefully fold in until the chunks are evenly distributed throughout the dough. Cover and chill in the fridge for at least 30 minutes - ideally overnight. I know it is tricky, but I promise it does make a difference and will be worth the wait!
When you are ready to bake the cookies, preheat the oven to 190°C/170°C Fan. Line a couple of baking sheets with non-stick baking paper and set aside.
Scoop the cookie dough into around 16 equal-sized balls and place roughly 5cm apart on the prepared baking sheets. Bake for about 11 minutes - or until the edges are just golden-brown but the centres are still puffy and slightly gooey. They might look underdone, but they will continue to bake as they cool on the trays.
Leave to cool before eating so they have time to firm up. Obviously they are delicious warm from the oven - but try to give them at least 15 minutes so you get the perfect chewy texture of the cookies.