Preheat your oven to 350°F (175°C). This ensures that your Monster Cookie Bars bake evenly and turn out perfectly chewy.
Grease a 9x13 inch baking pan or line it with parchment paper, leaving some overhang for easy removal later.
In a large mixing bowl, whisk together the all-purpose flour, oats, baking soda, and sea salt. This will help evenly distribute the leavening agent and salt throughout the flour.
In another bowl, use a hand mixer or stand mixer to cream together the unsalted butter, creamy peanut butter, brown sugar, and granulated sugar until light and fluffy. This should take about 2-3 minutes.
Add in the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract until combined.
Gradually add the dry ingredient mixture to the wet mixture, mixing until just combined. Be careful not to over-mix; you want to keep that chewy texture!
Gently fold in the semi-sweet chocolate chips and 1 cup of M&Ms. Reserve the remaining ½ cup of M&Ms for topping.
Pour the batter into the prepared baking pan, spreading it evenly with your spatula. Press the reserved M&Ms gently into the top.
Bake in the preheated oven for 25-30 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out mostly clean (a few crumbs are okay!).
Once baked, remove the pan from the oven and let it cool in the pan for about 10-15 minutes. Then, lift the bars out using the parchment paper overhang and allow them to cool completely on a wire rack. Cut into squares, sprinkle with flaky sea salt, and enjoy!
