Preheat the oven to 350°F. This is an important step, as it ensures the cookies bake evenly. Line two baking sheets with parchment paper to prevent sticking and make cleanup a breeze.
Beat together the butter, brown sugar, and granulated sugar in a large bowl or the bowl of a stand mixer until the mixture is light and fluffy. This should take about three to five minutes. The color should lighten as air is incorporated, resulting in a creamy consistency.
Add the egg, egg yolk, and vanilla. Mix until combined, ensuring the mixture is smooth and well-blended. This adds richness and flavor to the cookie dough.
Combine the dry ingredients: In a separate bowl, whisk together the all-purpose flour, cornstarch, baking soda, and salt. Gradually add this mixture to the wet ingredients, stirring until just combined. Avoid overmixing to keep the cookies tender.
Scoop the dough using a medium cookie scoop or a tablespoon, and place the dough balls onto the prepared baking sheets. Make sure to leave enough space between each ball for the cookies to spread during baking.
Press the centers gently with the palm of your hand. This helps the cookies to flatten slightly and ensures even baking.
Refrigerate the dough balls for 10 minutes. This step helps to firm up the dough, preventing them from spreading too much while baking.
Bake for 8 minutes, or until the edges are golden brown. The centers may look slightly underbaked, but they will continue to cook on the baking sheet.
Cool the cookies on the baking sheet for a few minutes before transferring them to a cooling rack. This allows them to firm up and makes them easier to handle.