Preheat oven to 350 degrees. Line a baking sheet with parchment paper or a silicone baking mat, set aside.
In a large mixing bowl with a hand-held mixer or the bowl of a stand mixer, add the butter. Mix on low for 1 minute until smooth. Add brown sugar and mix on medium until smooth and creamy, about 2 minutes.
Add egg, egg yolk and vanilla extract, mix on low until combined.
In a medium mixing bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt. With mixing speed on low, gradually add the dry ingredients, mix until combined.
Using a standard size cookie scoop, portion the dough and roll into smooth balls using your hands. Roll each cookie in granulated sugar, then place 12 cookies onto the prepared sheet.
Bake for 8-9 minutes or until set. The centers will be very soft but will set as they cool. Remove from the oven and allow to cool on the sheet for 5 minutes before transferring to a wire rack to cool completely. Bake remaining dough as instructed.
Store cookies in an airtight container at room temperature for up to 5 days.