- Preheat the oven to 350℉. 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.