Cut your cold butter into tablespoon sized cubes.
In a stand mixer, beat the butter on high speed with the brown sugar for 2-3 minutes. The mixture will be light and fluffy.
Add in 1 egg, then beat for one minute, then add your second egg.
In a separate bowl, whisk together the flour,
cornstarch, baking soda, and salt.
Fold the dry mixture into the wet mixture.
Cube up 10-12 Reese cups (I made them to be the same size as my chocolate chips)
Fold in Reese chunks & chocolate chips.
Refrigerate for at least 30 minutes (up to 24 hours)
Bake at 415 for 10-12 minutes.
Allow the cookies to rest on the baking tray for at least
10 minutes before transferring to a wire rack.