Halloween Monster Mash Cookies

Preheat the oven to 350° F. Line a baking sheet with parchment paper.In a bowl, beat together the butter, brown 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, and chocolate chips. Roll the dough into 1/4 cup size balls. Place 3 inches apart on the prepared baking sheet.Bake for 8 minutes, then rotate the pan and bake another 2 minutes. Pull the pan out and push the remaining M&M's and candy eyes 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.Eat warm or let cool and store in an airtight container for up to 4 days.