Heat oven to 350°F (180°C) with oven rack in middle (see Notes). Lightly grease cookie sheets.

Mix flour, cocoa, baking powder, baking soda, and salt together in a very large bowl.

Beat butter, granulated sugar, and brown sugar in bowl of a heavy-duty mixer on Medium-High speed until creamy, scraping down sides of bowl once or twice. Add vanilla and eggs and mix well.

Reduce mixer speed to Low and add the flour mixture. Scrape down sides of bowl and beat until dough forms.

Scrape dough back in to the very large bowl. Add the oatmeal, cornflakes, and chips and stir until well mixed.

Using a ¼-cup (59-mL) ice cream scoop (or cup measure), drop dough onto cookie sheets. Drop 6 cookies onto each cookie sheet.

Bake 15 to 20 minutes or until set in the center and edges are lightly browned. For softer cookies, reduce baking time slightly. The cookies will still look a little moist in the center. Cool cookies 1 minute on the cookie sheets and then place them on wire cooling racks.

These cookies should be stored loosely covered at room temperature.