In the bowl of a stand mixer, combine the sugars, peanut butter, butter and vanilla extract. Beat on high for 2 minutes until light and fluffy. Add the egg and yolk, mix until combined.
In a separate mixing bowl, whisk together the flour, baking soda, cornstarch and salt.
With mixing speed on low, gradually add the dry ingredients, mixing just until combined. Cover the bowl with plastic wrap and refrigerate for 15 minutes.
Meanwhile, preheat oven to 350 degrees. Line two cookie sheets with parchment paper or baking mats, set aside.
Using a standard size cookie scoop, create equal size balls of dough. Roll in the sugar and drop onto the prepared cookie sheets. Place in the oven and bake for 10-12 minutes, do not overbake! The cookies will look underdone... that's exactly what you want (I promise they won't be as they cool). Remove from the oven and allow to cool on the sheets for 3 minutes before carefully transferring them to a wire rack to cool completely.
Notes
-Rolling the cookies in sugar is completely optional. -Because these are not thick like traditional criss cross cookies, I wouldn't suggest adding any kind of chips, unless they are miniature chocolate chips.