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.
Next, add the egg and yolk, and mix until combined.
In a separate mixing bowl, whisk together the flour, baking soda, cornstarch, and salt.
With the 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 your oven to 350 degrees Fahrenheit (177 degrees Celsius) and, if needed, spray your mini muffin tin with non-stick cooking spray.
After it has chilled, roll the dough into 1-inch balls and then roll them in the additional granulated sugar. Place them into the inserts of the mini muffin tin.
Bake for 10 minutes until golden brown. While these are baking, be sure to unwrap enough bite-sized Reese’s Peanut Butter Cups for when the cookies come out of the oven.
Once you have removed the cookies from the oven, quickly place one Reese’s peanut butter cup into the center of each cookie and press down until at least half of the candy is inside the cookie dough.
Allow the cookies to cool before removing them from the tin.
Finally, serve and enjoy!