Begin by browning half the butter. Place into a small saucepan over low heat, and swirl the pan till the butter begins to bubble vigorously and turn an amber colour. You'll see flecks of brown appear on the surface, and the butter should release a nutty, caramel-like aroma. It will take around 5-8 mins.
Pour the butter into a large bowl, and let it cool slightly. Then add in the remaining butter.
Let the mixture sit for several minutes, till it reaches room temperature.
Add the sugars, and cream till light and fluffy, around 5 minutes.
Toss in the egg, egg yolk and vanilla extract, mixing till smooth.
Next, whisk in the salt and baking soda.
Fold in the flour, switching to a wooden spoon or spatula for ease.
Finally, fold in the chocolate chunks, till they are well-distributed.
Cover the bowl with plastic wrap and chill for 1-2 hours or overnight.
Preheat the oven to 180 C or 350 F and line a cookie sheet with parchment paper.
Remove the dough from the fridge. If it's too firm, let it sit for a few minutes to soften up. Otherwise, measure 60 g (2 and 1/2 tbsp) sized portions of the dough and roll into balls.
Place on the cookie sheet, around 2 inches apart (I do around 9 at a time) and bake for 12-15 minutes or until the cookies have spread out and the edges are golden. The longer you bake these the crunchier they will be.
Let the cookies cool on the baking tray for around 10 minutes, then transfer to a wire rack to cool completely.
These are delicious warm, out of the oven, with some ice cream for extra indulgence.