Line 2 baking sheets with parchment paper or silpat baking mats.
Whisk together the dry ingredients in a medium sized mixing bowl and set aside.
1 1/3 cups all purpose flour, spooned and leveled, 1 cup quick oats, 1 tsp. baking soda, 3/4 tsp. kosher salt
In a stand mixer fitted with the paddle attachment, cream together the butter, peanut butter, brown sugar, and granulated sugar on medium-high speed until light and fluffy, ~3-4 minutes, scraping down the sides of the mixing bowl as needed.
1/2 cup unsalted butter, softened to room temperature, 1/2 cup creamy peanut butter (like Skippy or Jif, not a natural peanut butter), 3/4 cup light brown sugar, packed, 1/2 cup granulated sugar
Add in the egg and yolk, milk and vanilla, mix again until combined, once again scraping down sides of the bowl as needed.
1 large egg, room temperature, 1 large egg yolk, room temperature, 1 Tbsp. whole milk, 2 tsp. vanilla extract
Slowly add in the dry ingredients, mixing just until combined. The dough will be really light/fluffy/airy but shouldn’t be overly sticky.
Add in the trail mix and fold in with a spatula, being careful not to over mix.
2 cups trail mix (I used a Mountain Trail Mix)
Let the dough rest at room temperature for 20 minutes or in fridge is OK too! This brief rest period will allow the flour to hydrate the dough and help us get a soft and chewy cookie.
Preheat the oven to 350°F. Scoop the cookie dough with a 2 Tbsp. scoop (should yield ~22-24 cookies).
Place on prepared baking sheets and top the cookies with a little more trail mix if desired.
Bake for 9-11 minutes (I like to go slightly underbaked for a delicious chewy cookie, the edges should be a light golden brown and centers still a little gooey looking, they will continue to bake on baking sheet when removed from oven). Swirl a cup around the cookies when still warm if needed to make perfect circles.
Let them rest on the cookie sheet for a few minutes then transfer to a wire rack to cool completely. Garnish with flaky sea salt and enjoy.