Heat oven to 350 degrees. Prepare a cookie sheet (12" x 17") by spraying with cooking spray.
In a large bowl, or bowl of stand mixer with the paddle attachment, combine butter, brown sugar, granulated sugar, and peanut butter. Mix until fluffy and pale in color. About 2-3 minutes.
Mix in the eggs and vanilla extract.
Add the baking soda, quick oats, chocolate chips, and m&m's. Mix to combine together.
** If wanted, reserve some chocolate chips and m&m's to press on top of the bars before baking.
Dump cookie dough onto the prepared cookie sheet and spread out. The dough is sticky so you may want to spray your hands with cooking spray to make it easier. You can also drop big spoonfuls of the dough all over the cookie sheet and then spread out. This makes it easier instead of doing one big pile in the middle of the cookie sheet.
Gently press the reserved chocolate chips and m&m's on top of the cookie bars.
Cook for 17-19 minutes. The edges should be golden brown and pulling away from the sides of pan, but the middle may still look slightly undercooked. That's ok. Let the bars cool in the pan for at least 30 minutes, and then cut into squares.
To make cutting the bars easy use a pizza cutter.
Some readers have reduced the sugars in this recipe with good results. I have always used the amount called for and we love it.