Preheat oven to 350 degrees. Line a large baking sheet with parchment paper.
In a large bowl, beat the butter and sugars until light and fluffy, about 2-3 minutes. Beat in the eggs, one at a time, until fully incorporated, then beat in the vanilla. Scrape down the sides of the bowl. Add the flour, corn flakes, oats, baking soda, and salt. Mix until just combined. Fold in the chocolate chips, M&Ms, and peanut butter chips until relatively evenly distributed. Do not overmix.
Use a 1/4 cup measure to scoop out portions of cookie dough and roll into a ball. The dough may be a touch sticky, but should not be gooey. Place no more than 5 dough balls to a cookie sheet. Bake for 8 minutes. Remove the cookie sheet and tap firmly on the counter 2-3 times to flatten. Return to the oven and bake 7-8 minutes longer until the edges are just set but the centers are still soft. Let cool on the baking sheet.
Serve the cookies warm, or cool completely and store, covered, for up to 5 days at room temperature or a week in the fridge.