Cut the butter into chunks and put it in a saucepan over medium heat, stirring occasionally until it has melted.
The butter will start to bubble and crackle, this is the water evaporating out of it. Keep stirring, once the crackling subsides, the butter will brown quickly, so don't take your eye off it. It will go from yellow to golden to a toasted brown color. It may froth up a little, so just use a spoon to scrape the froth to the side so you can check on the color.
As soon as it turns golden brown and you can smell a wonderful nutty aroma, remove it from the heat and transfer it to a bowl to cool completely to room temperature. You can either leave it sitting out to cool down gradually, or if you want to use it sooner, you can put the bowl over some ice and whisk it slowly until it cools down.
Make the Cookies
Once the brown butter has cooled, mix it together with the two sugars until combined.
Add the eggs and vanilla and stir them in.
In a separate bowl, combine the flour, baking soda, and salt and stir together. Add them to the other ingredients and mix them until incorporated.
Stir in the chocolate chips until well distributed. Be careful not to overmix.
Chill the dough for at least an hour, but preferably 24 hours. The longer it has to rest, the better the texture and flavor will be.
Preheat the oven to 350°F / 180°C (160° fan-assisted) and line a cookie sheet with parchment paper.
Scoop the dough into balls with a 1 ½-inch cookie scoop, or use a spoon and roll it into walnut-sized balls with your hands. Lay the cookie dough balls out on the cookie sheet with plenty of space between them.If the dough is too cold and firm, let it sit at room temperature until it is soft enough to scoop.
Bake the cookies for 12 minutes or until they are golden brown. You will probably have to do multiple batches as this recipe makes a lot of cookies!
The cookies will still be very soft, so let them sit on the baking sheet for 10 minutes before moving them to a wire rack to cool (or enjoy them warm!)
Notes
For best results, measure your ingredients with a kitchen scale, rather than cups. Cups are rarely accurate and can mess with your bakes.