BUTTER AND SUGARS: In the bowl of a stand mixer fitted with the whisk attachment (or use a large bowl and a hand mixer), add the room temperature butter (not melted, not softened) and dark brown sugar. Beat at medium speed until light in color, about 1-2 minutes. Add in the egg and vanilla and beat until light and fluffy, about 2 more minutes. Scrape down the sides of the bowl as needed.
BAKING AGENTS: Add in the baking soda and fine sea salt. Mix until just combined, about 20 seconds.
FLOUR, OATS, AND COCONUT: Add in the oats, flour, and coconut. Mix until just combined, about 30 seconds.
TRAIL MIX: Add in the trail mix and mix until ingredients are JUST combined. Do not overmix or overbeat. Overworking the dough will result in dense cookies. Cover the bowl of dough tightly and chill for 30 minutes.
SHAPE TALL COOKIE BALLS: Shape the cookie dough into balls -- a packed 2 tablespoons in size (if you have a food scale: 40 grams). Roll the balls taller rather than wider (see picture). Place them on a plate lined with parchment paper and cover gently with plastic wrap. Refrigerate for another 30-45 minutes. Don't refrigerate for too long. If you're in a rush, freeze the dough balls for about 20-25 minutes.
BAKE: Preheat oven to 325 degrees F. Line 2 half-sheet pans with Silpat liners (very important that the trays are lined, so the cookies don't burn on the bottom). Place 6 cookie dough balls per sheet pan (they can spread a good deal!), spacing them with plenty of room. Bake for 9 to 14 minutes (mine always take around 11-12 minutes), remembering they're best slightly underbaked. They should look ever-so-slightly gooey in the center (they harden a bit as they cool) and should be lightly browned around the edges. Remove from the oven (See Note 6 for troubleshooting) and if they're too puffy, bang the sheet pan a few times on the counter to get them to flatten more. Let stand on the cookie sheet for 5-10 more minutes to firm up enough to remove to a wire cooling rack. Right out of the oven you can press a few more chocolate chips or pieces of trail mix in the tops of the cookies if desired.
COOL AND ENJOY: Let cookies fully cool and set up before eating; they firm up a lot as they cool! Store in an airtight container at room temperature for up to 4 days. Cookies are best when eaten within 2-3 days.