Prep two large aluminum cookie sheets with parchment paper and set aside.
Preheat oven to 410 degrees F (yep, that's right!)
In a stand mixer fitted with a paddle attachment, whip up the butter for 1 minute until it's light and fluffy, then add in the sugars to cream together with the butter for 3 minutes on high speed.
Scrape down the sides of the bowl and then add in the vanilla and eggs one at a time, whipping up well between each addition.
Add in the flour, cornstarch, baking soda, and salt (no the sea salt yet - that's for sprinkling on at the end!) and slowly mix together by hand or on the very lowest mixer speed until just combined (don't overmix!)
Reserve 1/8 cup of add-ins, then hand-stir or lowest-speed-mix the add-ins into the batter. Split the dough into 8 balls (baseball-sized) and place 4 on each cookie sheet. Hand-press on the chocolate chips, M&M's, and chopped candy bars.
Bake for 9-10 minutes. They should be slightly golden, slightly mounded in the middle, thick/tall, and gooey in the center. Let them cool on the pan for 5 minutes, sprinkle with sea salt, then carefully transfer the cookies to a cooling rack to finish cooling. Enjoy!