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These easy Monster Cookie Bars combine creamy peanut butter, old-fashioned oats, chocolate chips, and colorful M&Ms in one delicious sheet pan dessert. Naturally gluten-free and perfect for feeding a crowd!

A batch of monster cookie bars topped with colorful candy-coated chocolates, cut into squares on parchment paper, with a large knife next to them on a wooden surface.


 

👩‍🍳 These cookie bars are perfect for feeding groups or taking to a bake sale — all the beloved flavors of traditional monster M&M cookies with half the work!

Table of Contents

Press Firmly: Really pack that cookie dough into your baking dish using clean hands or the back of a spatula. This prevents the bars from falling apart when you cut them. This makes a great handheld treat when they stay together!

Cool Completely: I know it’s tempting to cut into these beauties while warm, but letting them cool completely in the pan ensures clean cuts and prevents crumbling. Make them ahead at least several hours (maybe when the kids are at school and don’t know yet to be excited!)

Melissa in her kitchen smiling in front of the starting ingredients for the chicken the quinoa salad and skillet chicken on the wood block in front of her

🩷 Melissa

This sheet pan recipe yields generous bars that feed a crowd! Whether it’s a potluck, bake sale, or family gathering, these bars always disappear first.

One of my favorite things about these beauties is that they stay perfectly chewy at room temperature, making them ideal for packed lunches or picnics.

Since there is no flour in this recipe, these cookie bars are naturally gluten-free — another reason they’re great for taking along to a group event.

Don’t love M&M’s? While I adore the classic combination of mini chocolate chips and colorful M&Ms, you can use peanut butter chips, white chocolate chips, or even seasonal M&M colors for holidays.

A batch of monster cookie bars topped with colorful candy-coated chocolates, cut into squares on parchment paper, with a large knife next to them on a wooden surface.
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Monster Cookie Bars

By: Melissa Griffiths
These easy Monster Cookie Bars combine creamy peanut butter, old-fashioned oats, chocolate chips, and colorful M&Ms in one delicious sheet pan dessert. Naturally gluten-free and perfect for feeding a crowd!
Prep: 15 minutes
Cook: 22 minutes
Total: 37 minutes
Servings: 32 bars
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Ingredients 

  • ¾ cup granulated sugar
  • ⅓ cup brown sugar, packed
  • ¼ cup butter, softened
  • 1 cup creamy peanut butter, or chunky if you prefer
  • 1 large egg
  • 2 teaspoons vanilla extract
  • 1 teaspoon baking soda
  • ½ teaspoon salt
  • 3 cups old-fashioned oats
  • 1 cup M&M candies, (I prefer mini M&M's.)
  • 1 cup chocolate chips

Instructions 

  • Preheat the oven to 350°F (175°C). Grease a 9×13-inch baking dish and set aside. You can also line the baking dish with parchment paper if you'd like to remove the bars for serving.
  • In a large bowl, beat together the granulated sugar, brown sugar, softened butter, and peanut butter until smooth and creamy. Add the egg and vanilla, and mix until well combined.
  • Stir in the baking soda and salt, then mix in the oats until evenly distributed. Fold in the M&M’s and chocolate chips. I like to leave 1/3 of the M&M's out of the batter and save them to press on the top.
  • Press the dough firmly and evenly into the prepared baking dish. Sprinkle on extra M&M's if you saved them and press them in firmly into the top.
  • Bake for 22–26 minutes, or until the edges are golden brown and the center is set.
  • Let cool in the pan before slicing into bars. Enjoy!

Notes

Gluten-Free Treat: I LOVE that these are naturally gluten free, so keep that in the you back pocket for when you need to host your GF girlies! Be sure to use gluten free oats if that’s your goal.
Oat Choices: You can use old fashioned or instant oats or a combo of the two. The texture just changes a little bit depending on the oat you use. 
Switch Up Add-Ins: You can do all kinds of fun things with the M&M’s and chocolate chips. Use different kinds of M&M’s and chocolate chips to make this your own. Try a mix of peanut butter chocolate chips, normal chocolate chips, and peanut butter M&M’s. And it’s especially fun to coordinate the add-ins with the current season or holiday with colored or specially flavored chips and candies. SO good!
Good At All Temps: These firm up nicely when cooled and are soft and gooey when warm. I recommend eating them at all of the stages!

Nutrition

Serving: 1bar, Calories: 180kcal, Carbohydrates: 22g, Protein: 3g, Fat: 9g, Saturated Fat: 4g, Polyunsaturated Fat: 1g, Monounsaturated Fat: 3g, Trans Fat: 0.1g, Cholesterol: 10mg, Sodium: 124mg, Potassium: 95mg, Fiber: 1g, Sugar: 15g, Vitamin A: 66IU, Vitamin C: 0.04mg, Calcium: 23mg, Iron: 1mg
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Recipe Ingredients

Ingredients for baking are arranged on a countertop and labeled, including oats, sugar, brown sugar, eggs, mini M&M’s, peanut butter, vanilla, salt, baking soda, butter, and chocolate chips.
A glass mixing bowl with brown cookie dough and a wooden spoon on a white countertop, surrounded by a bowl of oats, a small bowl of white powder, and a partially visible bag of chocolate chips.

Step 1: Preheat the oven and prepare the pan. Beat together both sugars, softened butter, and peanut butter until smooth and creamy. Add the egg and vanilla, and mix until well combined.

A glass bowl with a mixture of oats and a brown, sticky ingredient being stirred with a wooden spoon, sitting on a white countertop.

Step 2: Stir in the baking soda and salt, then mix in the oats until evenly distributed.

A glass bowl filled with cookie dough mixed with oats, chocolate chips, and colorful candy-coated chocolates, with a wooden spoon resting inside.

Step 3: Fold in the M&M’s and chocolate chips. I like to leave 1/3 of the M&M’s out of the batter and save them to press on the top.

A rectangular baking pan filled with a baked oat bar mixture topped with colorful chocolate candies and chocolate chips, sitting on a white marble surface.

Step 4: Press the dough firmly and evenly into the prepared baking dish. Sprinkle on extra M&M’s if you saved them and press them in firmly into the top.

Cookie bars topped with colorful M&M candies, oats, and chocolate chips, creating a textured and vibrant dessert surface.

Step 5: Bake for 22–26 minutes, or until the edges are golden brown and the center is set.

A tray of colorful M&M cookie bars on parchment paper, with squares cut and served on white plates. A metal spatula rests on the tray, and a glass of milk sits nearby on a wooden surface.

Step 6: Let cool in the pan before slicing into bars. Enjoy!

Recipe FAQs

Can I use natural peanut butter in these monster cookie bars?

Natural peanut butter can work, though the texture might be slightly different. Just make sure to stir it well before measuring, as natural peanut butter tends to separate. If the mixture seems to dry and won’t hold together, you may need to add more peanut butter or another egg. It will just depend on the consistency of the natural peanut butter you use.

Can I substitute quick oats for old-fashioned oats?

Yes, you can use quick oats, but the texture will be slightly different – less chewy and more tender. Old-fashioned rolled oats provide that signature hearty texture that makes monster cookies so special.

What’s the best way to cut clean bars?

Use a sharp knife and wipe it clean between cuts. For extra clean lines, you can even run the knife under warm water and dry it between cuts. Cutting completely cooled bars also helps prevent crumbling.

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