In a mixing bowl, combine oil, sugars, egg, and vanilla on medium-high speed until it is light and fluffy.
Add in the flour, cocoa powder, cornstarch, baking soda, and sea salt and continue to mix on medium-low speed. Fold in the chocolate chips by and until just combined. *Note - do not blend the chocolate chips or Reese's Pieces with the mixer otherwise the chips will get crushed.
Shape dough into 1" balls and place on a large plate. Cover and chill in the refrigerator for 2 hours.
Preheat the oven to 350 degrees and place the dough balls 2" apart on a greased baking sheet.
Bake the cookies for 10-12 minutes (or until lightly browned) and cool on the baking sheet for 5 minutes. Then transfer the cookies to a wire rack to finish cooling.
These cookies can be eaten right away or placed in a freezer bag in the freezer for 3-4 months.