I made Oatmeal Monster Cookie Bars with a surprising pantry swap that makes the whole recipe unexpectedly simple.

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I made these Monster Cookie Bars because plain cookies were boring. They’re loud, messy, and kind of brilliant.

Imagine chewy bars studded with creamy peanut butter and plenty of M&Ms, each bite giving a little candy burst. I didn’t set out to perfect them, just throw stuff together and see what happens and somehow it became the thing people beg for at parties.

It’s not delicate, it’s a little reckless, which is why I love it. Try one and tell me if it’s too chaotic or totally addictive, I still can’t decide.

Why I Like this Recipe

– I love the chewy center and crispy edges, its totally addictive
– I can make a big batch and everyone always grabs seconds
– I dig that its forgiving so I can mess up a bit and still win
– It brings back fun, cozy memories and makes people smile

Ingredients

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Adds rich mouthfeel and moisture, mostly saturated fat, calories though, not a health food.
Creamy protein and healthy fats, adds peanut flavor and chew, can be salty.
Whole grain oats give fiber, chewiness, slow carbs and a more wholesome bite.
All purpose flour gives structure and rise, mostly starch, less nutrition than oats.
Brown sugar adds caramel notes and chew, white sugar boosts sweetness and crisp.
Provide bursts of sweetness, melting pockets of chocolate, extra sugar and little protein.
Extra crunch and savory salt, adds protein and healthy fats, watch portion sizes.

Ingredient Quantities

1 cup (2 sticks) unsalted butter, softened

1 cup creamy peanut butter

1 cup packed light brown sugar

1/2 cup granulated sugar

2 large eggs

2 teaspoons vanilla extract

2 cups old fashioned rolled oats

2 cups all purpose flour

1 teaspoon baking soda

1/2 teaspoon salt

1 cup M&Ms

1 cup semisweet chocolate chips

1 cup peanut butter chips

1/2 cup chopped roasted peanuts (optional)

How to Make this

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1. Preheat oven to 350°F (175°C). Line a 9×13 inch baking pan with parchment paper and spray or lightly grease the sides so bars dont stick.

2. In a large bowl cream 1 cup (2 sticks) softened unsalted butter, 1 cup creamy peanut butter, 1 cup packed light brown sugar and 1/2 cup granulated sugar until smooth and a bit fluffy.

3. Beat in 2 large eggs, one at a time, then stir in 2 teaspoons vanilla extract until combined.

4. In another bowl whisk together 2 cups old fashioned rolled oats, 2 cups all purpose flour, 1 teaspoon baking soda and 1/2 teaspoon salt.

5. Add the dry mixture to the wet and stir just until everything is blended. Dont overmix or the bars will get tough.

6. Fold in 1 cup M&Ms, 1 cup semisweet chocolate chips, 1 cup peanut butter chips and 1/2 cup chopped roasted peanuts if using. Reserve a handful of M&Ms or chips to sprinkle on top if you want them prettier.

7. Press the dough evenly into the prepared pan using a spatula or clean hands (lightly wet your hands so it smears less). Scatter the reserved candies on top and press down gently.

8. Bake 20 to 25 minutes until the edges are golden and the center is set but still moist. A toothpick should come out with a few moist crumbs not wet batter. For chewier bars pull them out a minute or two early.

9. Cool completely in the pan on a wire rack, at least 30 minutes, so the bars set up and cut cleanly. Lift out on the parchment and slice into squares.

10. Store in an airtight container at room temperature up to 3 days or freeze for longer.

Equipment Needed

1. Oven (preheat to 350°F / 175°C)
2. 9×13 inch baking pan plus parchment paper and nonstick spray or oil
3. Large mixing bowl and medium mixing bowl
4. Electric mixer (hand or stand) or a sturdy whisk
5. Rubber spatula (for folding and pressing) and a wooden spoon
6. Measuring cups and measuring spoons
7. Whisk or fork for the dry ingredients and eggs
8. Wire cooling rack
9. Sharp knife and small cutting board (for chopping peanuts) and a toothpick to test doneness

FAQ

Can I make these nut free?

Yes, swap the peanut butter for sunflower seed butter 1:1 and skip the peanut chips and chopped peanuts. Use allergy safe chocolate and M&Ms, but note the taste and color can be slightly different.

My bars turned out dry and crumbly, what went wrong?

Usually they were overbaked or the dough was too dry. Bake until the center is just set not rock hard, and press the dough firmly into the pan before baking so it sticks together better.

Can I use crunchy peanut butter or replace oats with quick oats?

Yup, crunchy PB works fine, it just adds texture. Quick oats can be used but the bars will be softer and less chewy since they absorb differently than rolled oats.

How do I keep the M&Ms and chips from sinking or bleeding color?

Reserve about 1/2 cup of the mix-ins and press them on top right after the bars come out of the oven. Use mini M&Ms if you can, they sink less and are less likely to bleed.

How should I store or freeze the bars?

Store in an airtight container at room temp up to 4 days, or in the fridge up to a week. For longer keep, slice and freeze in a single layer on a tray, then stack with parchment between pieces, freeze up to 3 months. Thaw at room temp.

Want chewier bars, not cakey. Any tricks?

Don't overbake, pull them when the center is just set. Also brown sugar helps chewiness so dont swap it all away. Cooling in the pan makes them finish and stay fudgy.

Monster Cookie Bars Recipe Substitutions and Variations

Butter (1 cup): swap with 1 cup solid coconut oil (use the scoop and level method) for a similar texture, or 1 cup unsweetened applesauce to cut fat but expect a softer, a little cakier bar. If using coconut oil, keep it cool so the dough isn’t too greasy.
Peanut butter (1 cup): use 1 cup almond or cashew butter 1 to 1, or 1 cup sunflower seed butter as a nut free option. For a lighter peanut flavor, try 3/4 cup powdered peanut butter rehydrated to about 1 cup.
Eggs (2 large): replace each egg with 1 tbsp ground flax mixed with 3 tbsp water (so 2 eggs = 2 tbsp flax + 6 tbsp water), or use 1/4 cup mashed banana or 1/4 cup applesauce per egg for egg free, moist bars.
M&Ms (1 cup): swap for 1 cup semisweet chocolate chips, chopped chocolate, Reese’s Pieces, or a mix of dried cranberries and white chocolate chips for a different sweet/salty pop.

Pro Tips

1) Measure the flour right: spoon it into the cup then level off with a knife. If you scoop it straight from the bag you’ll pack in too much and the bars get dry and tough.

2) Use true room temp butter and eggs. They cream together smoother so the texture is lighter. Dont overbeat once you add the flour or the bars will be tougher.

3) Press the dough evenly into the pan with slightly wet hands or a wet spatula so it doesnt stick and you get uniform thickness. Line the pan with parchment that has an overhang so you can lift the whole slab out cleanly.

4) Want chewier bars? Pull them out a minute or two early. Want cleaner slices? Cool completely then chill 30 minutes or freeze 10 to 15 minutes and slice with a hot, dry knife.

5) Save some M&Ms or chips for the top and press them on after the dough is in the pan or right after baking so the colors stay bright. Store in an airtight container, layering parchment between pieces so they dont stick.