Preheat an oven to 180c (350f) and prep a parchment paper-lined 9 by 9 baking pan.
Into a big mixing bowl, add the room temperature butter and peanut butter, and cream it with an electric hand mixer for 2-3 minutes or until it's creamy and light in color.
Add the brown sugar, granulated sugar, vanilla extract, and kosher salt. Mix until well combined.
Add the egg and egg yolk and mix until the mixture is creamy and smooth.
Add the rolled oats, flour, and baking soda and mix until almost combined.
Add the chocolate chips and m&m's and mix until just combined.
Transfer the cookie dough to a parchment paper lined 9 by 9 pan and top with more chocolate chips and M&M's. Bake in a 180c (350f) preheated oven for 25-30 minutes until the edges are golden brown.
Do not overbake it to keep it chewy in the middle!
Notes
Use room temperature ingredients. It is important that you use room-temperature ingredients like eggs and butter. This step ensures that your mixture mixes into a homogenous mixture and bakes perfectly.
Don’t overmix the batter, you don’t want to develop the gluten in the flour. This step ensures you have moist cookies and not dense and dry cookies.
Make sure that your butter is at room temperature. An hour or two before making these cookies, take the butter out of the fridge and let it come to room temperature. The butter should be soft to the touch. If your butter is too cold, it will be harder to mix and there will be lumps of butter in the dough.
Measure your flour correctly! When measuring your flour, avoid scooping it with a measuring cup. Instead, stir the flour with a spoon to create as many air pockets as possible, and spoon it into your measuring cup. Finally, level it with a back of a knife.
Line your baking pan with parchment paper! This step helps the cookie bars to bake evenly, and it makes sure that your bars release easily from the pan.
Be patient! I know it’s extremely tempting to eat these bars as they are pulled out of the oven, but let them cool for at least 30 minutes so they will be more firm and won’t crumble up.