Prep two large aluminum cookie sheets with parchment paper and set aside.
Preheat oven to 410 degrees (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 peanut butter, vanilla and eggs one at a time, whipping up well between each addition.
Add in the oatmeal, flour, cornstarch, baking soda, and salt (no the sea salt) and slowly mix together by hand or on the very lowest mixer speed until just combined (don't overmix!)
Reserve 1/8 cup of chocolate chips and mini M&M's, then hand stir or lowest-speed-mix the chocolate chips and mini M&M's 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 sprinkle on a touch of sea salt.
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, then carefully transfer the cookies to a cooling rack to finish cooling. Enjoy!