Prep Pans: Line two large baking sheets with parchment paper and set aside.
Mix the Dry Ingredients: In a medium size mixing bowl, sift together the flour, cornstarch, baking powder, and baking soda. Whisk in the salt and set aside.
Beat the Butter: In a large mixing bowl, using an electric hand mixer or a stand mixer fitted with the paddle attachment, beat the cold cubed butter for 30 seconds, just to break it up.
Add the Sugars: Add both sugars and beat for 1 minute, until combined. Scrape down the sides of the bowl.
Add the Eggs: Beat in the eggs one at a time, mixing for about 10 seconds after each addition. Add the egg yolk together with the second egg and mix just until incorporated.
Combine the Dough: Gradually add the dry ingredients in thirds, mixing on low speed until just combined.
Add Chia Seeds: Add the chia seeds and and gently fold in with a spatula until incorporated.
Portion the Cookies: Scoop dough into 2.8 oz. (80g / ⅓ cup) portions. Place 4-6 cookies per tray, leaving plenty of space between each cookie. You will need to bake them in batches.
Chill the Dough: Transfer the trays to the refrigerator and chill for 1 hour. This step is essential for thick cookies.
Preheat Oven: With 20 minutes of chill time remaining, preheat the oven to 400°F (200°C).
Bake: Bake cookies straight from the fridge for 12–13 minutes, or until the edges are lightly golden and set. The middles will look slightly underbaked.
Cool: Let cookies cool on the baking sheet for 10 minutes, then gently transfer to a wire rack to cool completely. Enjoy!