Preheat the oven to 350 F. Line a baking sheet with parchment paper and spray it before setting it aside.
In the bowl of a stand mixer fitted with a paddle attachment, mix the brown sugar, white sugar, butter and shortening. Beat on medium high speed for 3 minutes.
Add in the eggs one at a time, beating after each addition.
Add in the vanilla and mix to incorporate.In a lage mixing bowl, whisk together the flour, cornstarch, baking soda and salt.
Add the flour mixture to the wet mixture and beat on medium low speed just until the flour is incorporated. Fold in the chocolate chips.
Using a cookie scoop, scoop out a ball of cookie dough. Place it in the palm of your hand and shape it into a bowl shape.
Place a reeses cup, face down, into the center and fold the cookie dough over it. Pinch the dough together.
Place the cookie, face down, onto the baking sheet. Repeat this until your cookie dough is used. You should be able to fit about 6 cookies onto the baking sheet. They do not have to be spaced out very far as they do not expand.
Place into the oven to bake for 20-22 minutes, or until the edges of the cookies begin to slightly brown and the centers are baked.
Remove from the oven and let cool for 5 minutes before serving.
Storage: Cookies may be stored in a baggie or airtight container at room temp for up to 7 days or in the fridge for up to 10 days.
When forming the dough around the reeses, be sure to not get the dough too thin on one side and too thick on the other as the cookie will not bake evenly.
After the cookies have cooled and possibly set for a day, they are better when reheated for a couple seconds in the microwave.