Cook 6 tablespoons of unsalted butter in a small saucepan over medium heat until it turns golden brown and develops a nutty aroma (4 to 5 minutes). Remove from heat and add remaining butter until melted. Set aside.
In a medium bowl, whisk together 2 cups of all-purpose flour, 2 tablespoons of nonfat powdered milk, 1¼ teaspoons of fine grain sea salt, ½ teaspoon of baking powder, and ¼ teaspoon of baking soda. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, combine the melted butter with ¾ cup of light brown sugar and ½ cup of sugar. Beat on medium speed until loosely combined (30 seconds).
Add 1 egg and 1 egg yolk, along with 2 teaspoons of vanilla. Beat until combined (30 seconds).
Gradually add the dry ingredients in two additions, beating until just combined. Scrape down the sides as needed. The dough will be a little wetter than normal.
Fold in 8 ounces of bittersweet chocolate chips and 1 cup of toasted coconut flakes.
Transfer the dough to the refrigerator and let chill for 30 minutes.
Preheat the oven to 375°F and place an oven rack in the middle position. Line a baking sheet with parchment paper.
Use a scooper to portion out 1½-ounce rounds of dough and place them on the baking sheet, about 1 inch apart.
Sprinkle each cookie with a pinch of flaky sea salt and bake for 12 to 14 minutes until the edges start to brown.
Let the cookies rest on the baking sheet for 5 minutes, then transfer to a wire rack to cool.