Preheat oven to 350°F (176°C). Line 3 large baking sheets with parchment paper or silicone baking mats (or line one sheet and bake the cookies in batches).
In a medium bowl, whisk together the flour, cornstarch, baking soda and salt.
In a large bowl, using a stand mixer or hand-held mixer, beat the butter and both sugars together on medium speed until combined, about 2 minutes. Add egg, egg yolk and vanilla and beat until combined, scraping down the sides of the bowl as needed.
Beat in the dry ingredients on low speed until combined, then add the chocolate chips and stir to combine. The dough will the thick.
Divide dough into 6 even portions (each will be a little more than 1/2 cup). Shape into mounds, and place two on each baking sheet, at least 6 inches apart. Bake for 20-25 minutes, until edges and tops are lightly browned. The center will set as the cookie cools.
Let cookies cool on baking tray for 10 minutes then transfer to a wire rack to cool completely. Cookies will keep for up to 1 week stored at room temperature in an airtight container.