Combine butter and sugars in a large bowl and use an electric mixer to beat until creamy and well-combined.

Add peanut butter and stir again until smooth.

Add eggs and vanilla extract and stir until well-combined.

In a separate bowl, whisk together flour, cornstarch, baking soda, and salt.

Add your flour mixture to your wet ingredients and stir (with mixer on low speed so that you don't send flour flying everywhere!) until flour is completely incorporated into the batter. Be sure to scrape the sides and bottom of the bowl so that ingredients are well-combined.

Add oats and stir well.

Cover your cookie dough with plastic wrap and transfer to the refrigerator. Allow dough to chill for at least 30 minutes (this dough may be chilled overnight if needed).

While dough is chilling, preheat your oven to 375F (190C) and line cookie sheets with parchment paper.

Once dough has chilled, remove from the refrigerator and roll into 1 ½ Tablespoon-sized balls and place on prepared cookie sheet, spacing cookie dough at least 2" apart.

Bake on 375F (190C) for 10 minutes or until edges are just beginning to turn golden brown. Allow cookie dough to cool completely on cookie sheet before enjoying (the centers will continue to bake as they cool).