Preheat your oven to 350°F and line two baking sheets with parchment paper.
In a medium bowl, whisk together the quick oats, baking soda, and set aside.
In a large mixing bowl, combine the softened unsalted butter, granulated sugar, and brown sugar. Beat until the ingredients are creamy and well combined.
Beat in the peanut butter followed by the vanilla extract and the eggs and mix until incorporated.
The next step is to gradually add the dry oat mixture to the peanut butter mixture and mix until the oats are moistened.
Stir in the Halloween M&M's and the semisweet chocolate chips.
Using a #40 cookie scoop or your hands, form 1.5 tablespoons worth of dough into balls and place them on the prepared baking sheet.
Place the balls of cookie dough two inches apart on the cookie sheet and gently press the tops of the cookie dough balls slightly to flatten them.
Bake the cookies one sheet at a time in the preheated oven for about 10-12 minutes, or until the edges are slightly golden brown.
Remove the cookies from the oven and let them cool on the baking sheet for about 5 minutes before transferring them to a cooling rack to cool completely.
Once cooled completely, transfer the monster cookies to a serving platter if serving right away or store them in an airtight container for maximum freshness.
Notes
Use chunky peanut butter. Creamy works but the texture is softer.
Do not overbake. Pull them when the centers look slightly soft.
Flatten the dough balls before baking. Monster cookie dough does not spread much.
Store in an airtight container on the counter for up to 5 days.
Freeze dough balls for up to 3 months. Bake from frozen and add a minute.