Monster Cookie Bars are the ultimate cookie mashup because they are chock full of oatmeal, peanut butter, M&Ms, and chocolate chips! Soft and chewy confections and one-bowl recipe that will become a family favorite in no time!
The cooling temperatures this week has gotten me pretty nostalgic for comforting ingredients and over the top cookies chocked full of surprise ingredients. Monster bars definitely fit the bill!
They are sweet, salty, easy to make and hard to resist. This delicious confection is a hybrid of your favorite Oatmeal, Peanut Butter, and Chocolate Chip Cookie all rolled into one. Need I say more?
And this recipe is a huge timesaver! Made in one bowl, there is no rolling out dozens of cookies or waiting for batches to cool. Last minute cookie exchange recipe duty? This is a recipe you’ll want in your lineup because they are easy to make and a crowd favorite.
The bars are soft and chewy, thanks to the oats and creamy peanut butter. For years chocolate and peanut butter has reigned supreme as the dynamic dessert duo, and this crowd favorite highlights exactly why.
So, gather your own little cookie monsters, and let’s indulge in this dynamic quad-fecta. With a pinch of magic, it’s also super easy to make holiday festive with substitutions like sprinkles or red and green candies. Keep reading for more ways to customize this very simple cookie recipe.
Ingredients and Notes
Below is an overview of what you will need to make these with notes for potential substitutions.
Flour: I used all-purpose flour in this recipe. These cookie bars also work well with a gluten free flour.
Brown Sugar: Dark brown sugar tends to have a slightly stronger flavor and higher moisture content, so I generally use light brown.
Old-fashioned Oats: Use Old-fashioned oats in this recipe. Quick oats cook much more quickly and cause your bars to have a doughy consistency. If you plan to make this gluten free, make sure you are using gluten free oats.
Chocolate Chips: Use whichever brand of chocolate chips or chunks you prefer.
Peanut Butter: I recommend a creamy variety, like Jif or Skippy, as it adds flavor but not more texture. However, either do work if you like the crunch. I also don’t recommend all-natural varieties because they tend to separate in baking or can be unpredictable. Make sure to give the jar a good stir before pulling out the ½ cup.
Butter: Your butter should be softened at room temperature, but not melted. I always use unsalted and add salt as needed, but salted butter should be fine.
Add-In Ideas for Monster Cookies
One of the best things I love about this recipe is that it’s great to make when I need to clean out the pantry and have a little of a bunch of different ingredients! Feel free to substitute some of the M&Ms or chocolate chips for:
- Milk chocolate chips, white chocolate chips, or butterscotch chips.
- Peanut butter candies like mini-Reese’s cups or pieces.
- Cut larger candies in half or quarter.
- Pretzels or shredded coconut
- Put sprinkles or sanding sugar over the tops just before baking.
- Cereal like fruity pebbles, cinnamon
- Heath or toffee bits or rolo candies.
- Halloween – leftover candy bars (edible candy eyes after baking make this truly monster!)
- Christmas monster cookies – use red and green M&M’s and/or add red and green sprinkles
- Valentine’s – use red, pink, and white M&M’s and/or sugar hearts
- Easter or Spring – use pastel colored M&M’s or mini robin’s eggs
Step by Step Recipe Guide
This recipe is about as fast and easy as it gets! The full recipe is at the bottom of this post, but I have a few tips to impart:
DO make sure to use a greased or lined 9×13 pan for best thickness. The recipe can be cut in half for an 8×8, but sizes in between can produce an overly thick, chewy bar. I like to use parchment paper cut wider than the longer sides. It acts like handles and the whole bars are easy to lift out.
Cream butter and brown sugar. If your butter is room temperature, the mixture should look smooth. If you have bits or chunks of butter, wait a few minutes to warm and mix again. (left photo above)
Combine other ingredients to form dough. Add in flour, oats, peanut butter, and baking soda. I just dump it all in and then mix until just combined. You don’t want to over-mix here. (right photo above)
Add mix ins. Using a large wooden spoon or spatula, stir in chocolate chips and half of the M&M’s.
Spread out in the baking pan. Take the rest of the M&Ms or other candies and sprinkle over the top for additional color. Press them into the dough just a little bit and bake at 350 until the edges darken a little (about 18 minutes).
- Make sure to let the bars cool before cutting. I know, it’s hard! The chocolate and m&ms tend to smear.
- This recipe can be divided in half and prepared in an 8×8 pan for a smaller batch.
- If making the full recipe, I highly recommend using a deep 9×13 inch baking pan. The bars are thick already and this sizing ensures they cook properly and evenly. Anything less will make a thicker, doughier bar.
Store: These Monster Cookie Bars can be stored in an air-tight container at room temperate for 3-4 days.
Freeze: You may freeze these bars to enjoy for later and they should last about 2 months. I recommend cutting and wrapping the bars individually to prevent sticking.
Make ahead dough: The dough can be prepared and stored in the refrigerator up to 48 hours ahead of time. It also freezes well. Make sure it is in an air-tight container and brought to room temperature on the counter before baking.
More Fun Dessert Recipes from Our Kitchen
Quick oats can certainly be used if that is all you have on hand or prefer a more doughy bar. I like the texture of old fashioned oats in this recipe as they have a more rustic, hearty bite.
Yes! These bars can easily be made gluten free by swapping out with your favorite gluten free flour. King Arthur & Bob’s Red Mill are a couple popular brands that can be used with a 1:1 ratio. Oats are generally gluten free, but make sure the packaging states they are made in a gluten free facility if you have an allergy.
You can substitute or add in whatever nuts or candy you prefer. Reese’s Pieces can be used in place of M&M’s for more peanut butter flavor and pretty Fall colors. Peanut butter chips, white chocolate chips, nuts, or toffee bits are also some alternative add-ins that you might enjoy.
The dough can be prepared and stored in the refrigerator up to 48 hours ahead of time. Make sure that the prepared dough is sealed in an air-tight container and brought to room temperature on the counter before baking.
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Monster Cookie Bars
Oatmeal Cookie Bars
- ½ cup Unsalted Butter softened
- 1 cup Light Brown Sugar
- 1 teaspoon Vanilla Extract
- 2 Eggs
- 2 cup All-Purpose Flour
- 2 cup Old-fashioned Oats
- ½ cup Peanut Butter Creamy or Crunchy
- 1 teaspoon Baking Soda
- ½ teaspoon Salt
- 1 cup Chocolate Chips about 6 ounces
- 1 cup M&Ms
- Preheat oven to 350 degrees F. Grease a 9×13 deep pan and set aside.
- In a large mixing bowl, combine butter, brown sugar, and vanilla until smooth. Break eggs and beat until smooth. Add in flour, oats, peanut butter, baking soda, and salt. You can use a stand mixer or electric mixer or hand mix.
- Using a large spoon, stir in the chocolate chips and half the M&Ms to the batter.
- Press batter into greased baking dish. Sprinkle remaining M&Ms on the top, pressing them into the dough slightly.
- Bake for 15-18 minutes or until the edges crisp and slightly brown or a toothpick comes out clean. Let cool a bit before slicing. Letting them cool completely is best, but who could wait that long!