Preheat oven to 350°F (180°C). Line rimmed baking sheets with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugars at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Reduce mixer speed to medium-low. Add egg and egg yolk, one at a time, beating well after each addition. Beat in vanilla.
In a medium bowl, whisk together flours, cornstarch, baking soda, and salt. Gradually add flour mixture to butter mixture, beating just until combined (do not overmix). Gently stir in chocolate morsels. Using a 1-tablespoon scoop, drop dough at least 2 inches apart onto prepared pans. Place an additional small (1½ teaspoons) scoop of dough onto each cookie. Lightly press together.
Bake until edges are just beginning to brown, 9 to 10 minutes, rotating pans halfway through baking. (Cookies may look under baked, but this helps achieve a soft and chewy cookie.) Bake sheet at a time for even cooking. Let cool on pans for 5 minutes. Using a thin spatula, remove from pans, and let cool completely on wire racks. Store in an airtight container at room temperature for up to 3 days.
Notes
*We used Kerrygold and Guittard.
Recipe by Bake from Scratch at https://bakefromscratch.com/chewy-chocolate-chip-cookie/