In a small saucepan, melt the butter over medium heat- until it comes to a boil. Swirl the pan occasionally until brown flecks begin to appear on the surface of the butter, and the liquid begins to turn an amber color. It should give off a nutty, caramel-like aroma after about 5 to 8 minutes. It is better to undercook rather than overcook the butter, since it goes from brown to burnt quickly once it reaches the right color.
Take the butter off the heat and pour it into a large bowl, then let it cool to room temperature.
MAKE THE COOKIE DOUGH:
Stir the sugars into the butter. Then stir in the egg and vanilla extract. Once you add the egg, the mixture should thicken. Next, stir in the flour, salt and baking soda. Stir until well-incorporated. Finally, fold in the nuts and chocolate chunks.
Cover the bowl with plastic wrap and chill in the refrigerator for at least 30 minutes or up to 2 days. Chilling the dough makes the flavors more prominent and makes it easier to shape the cookies.
BAKE THE COOKIES:
Preheat the oven to 350 degrees F. and line a baking tray with parchment paper.
Roll the dough into 1½-tablespoon-sized balls and place about 2-inches apart on the tray.
Bake for 9 to 10 minutes or until the edges are golden and the center is just set.
TIP: It's better to underbake these cookies by a minute rather than over-baking them. They will be chewier and softer, rather than crisp. Sprinkle the warm cookies with flaky sea salt. Let them cool.
Notes
To store: place in a sealed container and keep at room temperature for up to 2 weeks. Freeze for up to 3 months.
To keep cookies soft, store them with a slice of bread overnight. They will soften as they soak up the moisture in the bread.