Preheat your oven to 350°F (175°C). Line several large baking sheets with parchment paper. These cookies are big, so you'll need the space!

In a very large bowl (or the bowl of a stand mixer), beat the softened butter, brown sugar, and granulated sugar together on medium speed until creamy and well-combined, about 2-3 minutes.

Beat in the creamy peanut butter until smooth. Then, add the eggs one at a time, followed by the vanilla extract, mixing well after each addition.

Sprinkle the baking soda over the mixture and mix it in. On low speed, gradually add the rolled oats, mixing until they are just incorporated. The dough will be very thick and heavy.

Using a wooden spoon or spatula, fold in the chocolate chips and M&M's. Be careful not to overmix.

Use a large cookie scoop or a ¼ cup measure to portion out the dough. Roll into balls and place them on the prepared baking sheets, leaving at least 3 inches between each cookie as they will spread. Gently press a few extra M&M's onto the tops of the dough balls for a more colorful appearance.

Bake for 12-15 minutes, or until the edges are set and lightly golden. The centers will look soft and a bit underdone. This is key to a chewy cookie!

Let the monster cookies cool on the baking sheets for at least 5-10 minutes before attempting to move them, as they will be very soft. Transfer carefully to a wire rack to cool completely.