Preheat the oven to 350° F. Line a baking sheet with parchment paper.
In a large mixing bowl, beat together the butter, brown sugar, granulated sugar, and vanilla until combined.
Beat in the eggs and 1 egg yolk, 1 at a time, until combined. Add the flour, baking soda, baking powder, and salt. Beat in the oats.
Fold in 1 cup of the M&M's, the pretzels, chocolate chips, and peanuts.
Roll the dough into 1/4 cup size balls. Place 3 inches apart on the prepared baking sheet.
Transfer to the oven and bake for 8 minutes, then rotate the pan and bake another 2 minutes. Pull the pan out and push the remaining M&M's into the cookies. Bake 2-3 minutes.
Remove from the oven and let the cookies cool on the baking sheet. They will continue to cook slightly as they sit on the baking sheet.