Brown The Butter. Melt the butter in a light-colored saucepan over medium heat, whisking constantly until the butter begins to foam. (Watch carefully when the butter begins to foam, as it will turn golden and brown quickly.) When the butter under the foam is golden brown (like honey) and smells sweet with a nutty aroma, remove the pan from the heat and transfer to the bowl of your stand mixer or a mixing bowl to cool for 20-30 minutes, until it’s cooled to room temperature. (Make sure to get all those lovely brown bits from the bottom of the pan!)
Combine Butter & Sugars. Add brown sugar and granulated sugar to the ROOM TEMPERATURE brown butter (don't add if it's still hot!). Mix with the paddle attachment or an electric mixer 1-2 minutes, or until the mixture looks like wet sand.
Add Eggs + Egg Yolk One At A Time. Add eggs + egg yolk one at a time, blending well after each addition.
Build Flavor. Add the vanilla extract and coconut extract, if using, and mix to combine.
Add Dry Ingredients. Add flour, cornstarch, baking soda, baking powder, and salt. Mix until just a few floury patches remain, then add the chocolate chips and shredded coconut. Mix until the coconut and chocolate chips are evenly mixed throughout the dough and there are no more dry spots left.
Scoop & Chill The Dough. Use a large cookie scoop (3 Tablespoon size) to scoop cookie dough balls. (You'll get about 20-22 cookies.) Place in an airtight container, cover, and chill the cookie dough balls for at least 2 hours, up to 1 day in advance.
Preheat The Oven & Prep Your Pans. When you’re ready to bake, remove the dough from the refrigerator while the oven fully preheats to 350 degrees F. Line two baking sheets with parchment (so you can alternate sheets between batches.)
Bake & Cool The Cookies. Roll 6 cookie dough balls in between your palms to smooth them out and place onto a prepared baking sheet. Bake at 350 degrees for 10-12 minutes, or until the edges are a golden brown color and the centers look just underdone. Cool on the baking sheet for 2 minutes before transferring to a wire rack to finish cooling. Repeat process with remaining cookie dough.
Serve & Store. Enjoy right away once the cookies are cooled down, or store in an airtight container 2-3 days. (Or, freeze up to 2 months)