Preheat the oven to 350 degrees F. Generously grease a 9x9-inch pan with nonstick spray.
Use a handheld electric mixer to cream together peanut butter and butter in a large bowl. Beat in the brown sugar, granulated sugar, eggs, egg yolk, and vanilla extract.
Add the oats, flour, baking soda, and salt, and stir to combine. Stir in the chocolate chips and M&M's candy.
Transfer the dough to the prepared pan and spread it out evenly. Sprinkle a few more M&M’s on top.
Bake until the bars are golden, puffed and set- about 30 minutes. They will be more set on the outside than the inside, and will set more as they cool. Like brownies, be careful not to over-bake.
Cool slightly before cutting and serving.