Heat oven to 350°F (180°C) with oven rack in middle position. Line 2 baking sheets with parchment paper or Silpat.
Mix the flour, cocoa, baking powder, baking soda, and salt together in really, really, really large bowl or a dishpan.
Beat the butter, granulated sugar, and brown sugar in bowl of a mixer on medium-high speed until creamy, scraping down sides of bowl once or twice. Add the vanilla and eggs and mix well.
Reduce the mixer speed to low and add the flour mixture. Scrape down the sides of the bowl and beat just until a dough forms. Scrape the dough back into the very large bowl. Dump in the oatmeal, corn flakes, and M&Ms or chips and stir with a spoon until combined.