1. Prepare Your Pan: Line an 8×8-inch baking pan with parchment paper, leaving an overhang on the sides. This will make it incredibly easy to lift the bars out once they’ve set. Lightly grease the parchment paper if desired, though it’s often not necessary with parchment.

2. Cream the Wet Ingredients: In a large mixing bowl, combine the softened peanut butter, softened unsalted butter, light brown sugar, granulated sugar, and vanilla extract. Beat with an electric mixer on medium speed until the mixture is light, fluffy, and well combined, scraping down the sides of the bowl as needed. This step is crucial for creating a smooth base.

3. Add the Oats: Reduce the mixer speed to low and gradually add the quick-cooking oats to the peanut butter mixture. Mix until just combined, being careful not to overmix. The mixture will be thick and somewhat crumbly at this stage.

4. Fold in the Mix-ins: Remove the bowl from the mixer. Add the semi-sweet chocolate chips and M&M’s candies to the oat mixture. Use a sturdy spatula or wooden spoon to gently fold them in until they are evenly distributed throughout the dough. You want every bite to have a good mix of flavors!

5. Press into the Pan: Transfer the entire mixture into the prepared 8×8-inch baking pan. Using the back of a spoon, a flat spatula, or even your clean hands, press the mixture down firmly and evenly into the pan. Make sure it’s compacted well from edge to edge to ensure the bars hold together nicely.

6. Chill to Set: Place the pan in the refrigerator and chill for at least 30 minutes, or until the bars are firm enough to slice. This chilling time is essential for the bars to set properly and achieve their desired chewy texture.

7. Slice and Serve: Once thoroughly chilled, use the parchment paper overhang to lift the entire slab of bars out of the pan. Place it on a cutting board and slice into 16 squares (4×4 grid) or desired bar size. Serve immediately and enjoy your delicious flourless monster cookie bars!