Heat the butter in a saucepan over medium-heat. While stirring frequently, allow the butter to cook. The butter will begin to bubble and foam a bit, continue stirring. Remove from heat when you notice the formation of small brown specks, and the liquid changing to a deep golden colour. You will also notice a nutty aroma.
Once removed from heat, at ice-cubes one at a time to the butter. This is replacing the moisture that you just boiled off. Set aside to cool.
Make the dough
Combine brown sugar, granulated sugar, salt and baking soda in a medium bowl.
Add the melted butter and whisk to combine. Add egg and vanilla, and whisk for a few minutes until the mixture to glossy and smooth.
Gently fold in the flour until no large streaks remain - do not over mix! Add in chocolate chunks and fold in until evenly distributed.
Form dough into 12 balls (a bit larger than golf ball size).
Cover/wrap the dough balls and refrigerate. Let chill for at least 1 hour (but the longer the better).
Bake the cookies
Preheat oven to 350F. If you prefer thinner cookies, remove your dough balls from the fridge while the oven is preheating.
Arrange the dough balls on a silicone mat or parchment paper on your baking tray. Top the cookie dough with coarse sea salt flakes (optional).
Bake cookies for 12-15 minutes, until golden brown around the edges.
Enjoy :)
Notes
Brown butter - If you do not want to make the brown butter, no worries, simply melt the butter in the microwave. I have to say though, the brown butter adds a depth of delicious nutty and rich flavour.
Chocolate chunks - I find using chopped chocolate from a bar creates better melting and pooling of the chocolate in your cookies. Chocolate chips totally work as a replacement.
Refrigeration time - If you can wait, I recommend letting the dough rest in the fridge for 24-72 hours. This allows the flour to absorb more moisture, improving the cookies texture, and develop deeper flavour as the ingredients sit together.