Inspired by monster cookies, these monster cookie bars are chewy peanut butter oatmeal bars packed with M&M candies and chocolate chips. They’re made in one bowl and baked in under an hour!
Monster cookies have always been a top seller at every bakery I’ve worked at, which is why I created these fun and easy monster cookie bars! There is no dough chilling or scooping—like with my M&M cookies—just pressing the dough into a pan and baking.
They’re thick, chewy, and loaded with gooey chocolate, tons of M&Ms (just like my monster cookie cupcakes and M&M brownies), oats, and peanut butter. They’re completely customizable and perfect for any occasion!
Table of Contents
Best Mix-Ins
M&Ms: You can use mini or regular M&Ms or a combination!
Chocolate chips: A must-have for that gooey, melty texture. You can mix semi-sweet, milk, white, or dark chocolate chips!
Peanut butter chips: You can replace the regular chocolate chips with peanut butter chips or Reese’s pieces for even more peanut butter flavor!
Pretzels: Crushed pretzels can give the cookie bars a sweet, salty flavor and crunchy texture.
3 Tips for Perfect Cookie Bars
1. Don’t overmix the cookie dough. Mix just until the ingredients are combined. Overmixing can make them tough instead of soft and chewy.
2. Bake until golden brown. For a gooey and chewy texture, bake until the top is golden brown and puffy. A toothpick inserted into the center should come out slightly moist but not covered in thick, wet dough.
3. Cool completely before cutting. Allow the cookie bars to cool in the pan before cutting into them. This helps the middle set and prevents them from falling apart!
Endless Ways to Customize
Spooky Halloween treats: Press eyeball candies on top or use leftover Halloween candy.
Game day colors: Use M&Ms in your team colors—go sports!
Festive for Christmas: Mix in red and green M&Ms and drizzle with white chocolate for a holiday treat.
Valentine’s Day shades: Add pink and red M&Ms and heart sprinkles!
Choosing the Right Baking Pan
Choose a light-colored metal pan: Glass, ceramic, or dark-colored pans conduct heat differently and require different bake times!
Opt for a 9×13-inch pan: This size is ideal for the monster bars, giving them the perfect thickness for a chewy texture without being too thick or thin.
Ingredient Notes and Substitutions
Peanut butter: Similar to my nutter butter bars, natural, creamy, no-stir peanut butter is best for this recipe! Other varieties have added sugars, oil, and salt, which can affect the overall flavor and texture. Use almond butter in substitution!
Oats: For the best texture, use rolled oats (also known as old-fashioned oats). You can use quick oats, but the bars will have less of an oatmeal texture.
Recipe Instructions
Step 1: Cream the butter, sugar, and peanut butter. In a large mixing bowl with a hand mixer or stand mixer, cream the peanut butter, softened butter, granulated sugar, and brown sugar until light and fluffy.
Step 2: Mix in the wet ingredients. Mix in the egg, egg yolk, and vanilla extract until combined. Scrape down the bowl if necessary.
Step 3: Mix in the dry ingredients. Use a rubber spatula to fold in the flour mixture, M&Ms, and chocolate chips.
Step 4: Bake! Evenly press the cookie dough into the prepared baking pan. Sprinkle more M&Ms and chocolate chips over the top, then bake until the top and edges are golden brown. Let the pan cool on a wire rack before cutting into squares!
Top Tips for Bakery-Style Results
Use a kitchen scale. Baking with a scale is my #1 baking tip! If you do not have a scale, use a spoon to fluff the flour first, then spoon it into your measuring cup.
Use room temperature ingredients. Let your butter and eggs sit out until they’re at room temperature. This helps them mix better, giving you soft and chewy cookie bars.
DIY Gift Giving
Turn this recipe into the perfect DIY gift by layering the dry ingredients in a jar! It’s a thoughtful and fun present that anyone will love. Layer the oats, sugar, M&Ms, chocolate chips, and other dry ingredients into a jar and attach a cute label with baking instructions. Perfect for holidays, birthdays, or just because!
Storage and Freezing
Storage: Leftover bars can be stored in an airtight container at room temperature for 2-3 days. I don’t recommend refrigerating them, as they will dry out.
Freezer: You can freeze the entire pan or individual bars in a freezer bag or airtight container for up to 3 months.
FAQs
You can substitute the peanut butter with almond butter!
This is almost always due to either using too much flour or overbaking.
Yes! Cut the recipe in half and bake it in a square 8×8 baking pan for about 20-25 minutes.
More Cookie Bar Recipes
If you tried this or any other recipe on my website, please let me know how it went in the comments; I love hearing from you! Also, please leave a star rating while you’re there! You can also tag me on Instagram or Facebook so I can check it out!
Monster Cookie Bars
Equipment
Ingredients
- 14 tablespoons unsalted butter , room temperature
- 1/2 cup creamy peanut butter
- 1/2 cup light brown sugar, packed
- 1/2 cup dark brown sugar, packed
- 1/2 cup granulated sugar
- 2 large eggs, room temperature
- 2 large egg yolks, room temperature
- 1 tablespoon vanilla extract
- 2 cups all-purpose flour, *See notes below for measuring*
- 1 3/4 cup rolled oats (old-fashioned oats)
- 1/2 cup semi-sweet chocolate chips
- 1/2 cup chopped chocolate bar
- 1/3 cup mini M&Ms
- 1/3 cup regular M&Ms
- 1 teaspoon baking soda
- 1 teaspoon fine sea salt
Instructions
- Line a 9×13 baking pan with parchment paper or grease the sides and bottom with nonstick spray or butter. Then preheat the oven to 350 F/180 C.
- In a mixing bowl, cream the butter, peanut butter, brown sugar, and granulated sugar until light and fluffy.14 tablespoons (196 g) unsalted butter, 1/2 cup (132 g) creamy peanut butter, 1/2 cup (100 g) light brown sugar, 1/2 cup (100 g) dark brown sugar, 1/2 cup (100 g) granulated sugar
- Mix in the eggs, egg yolks, and vanilla.2 (100 g) large eggs, 2 (36 g) large egg yolks, 1 tablespoon vanilla extract
- Fold in the flour, oats, baking soda, salt, chocolate chips, chopped chocolate, and M&Ms until just combined. Do not overmix! Evenly spread the cookie dough into the prepared baking pan. Sprinkle the top with extra chocolate chips, chopped chocolate, and M&Ms if desired.2 cups (260 g) all-purpose flour, 1 3/4 cup (180 g) rolled oats (old-fashioned oats), 1/2 cup (85 g) semi-sweet chocolate chips, 1/2 cup (78 g) chopped chocolate bar, 1/3 cup (75 g) mini M&Ms, 1 teaspoon baking soda, 1 teaspoon fine sea salt, 1/3 cup (78 g) regular M&Ms
- Bake in the preheated oven for 25-35 minutes or until the top is golden brown and a toothpick inserted into the center comes out just barely moist. Let them cool completely on a wire rack before cutting then enjoy!
Video
Notes
Tried this recipe?Leave a comment below & tag @cambreabakes on social!
Made these for the first football Sunday & my friends and family were so obsessed! It is literally all the things, chewy, chocolatey, full of peanut butter & candy! I love that it is an oat based bar, it gives it an amazing texture. Will definitely be making these again!
Hi! I was looking for your recipe for dirt pudding brownies as I was planning on making it but it seems to not be on the blog anymore! Are you reworking it?
Hi Maya! Unfortunately, that recipe was lost in a few website transitions and it is now lost to the web 😢
Sorry, I am confused about the metric measurements, 1-2-3 ?? If it’s all for the
same 8 + 8 size baking pan ? Please help, as I would love to make them
for my grandkids ?! I need everything in cup measurements. Also, why 2 or 3 eggs ?
The 1-2-3 is strictly for doubling or tripling the recipe. It’s default is 1x. If you want everything in cup measurements don’t click the metric button. There is only 1 egg in the recipe, not 2-3.
All of your recipes are amazing but i have to say this one is my new favorite!!! SOOO GOOD 🤤
Stop what you’re doing and make these immediately!!! Sooooo delicious, chewy, soft, chocolatey, just perfect.