Monster Cookie Bars | My Baking Addiction

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Monster cookie bars couldn’t be easier to make. This sweet treat is a great way to use up leftover candy and bakes up in no time to make your littlest monsters happy!

We’re just over a month away from Halloween, but I feel like I’ve been seeing Halloween candy at the store for weeks now.

I’m not the only one, right?

With Halloween decorations appearing around the neighborhood and the new Hocus Pocus movie coming out, it’s time to start planning for that post-Halloween problem:

What do we do with all of this leftover Halloween candy?

These monster cookie bars are the perfect solution if you ask me. Whether you’ve got an excess of mini M&M’s or candy bars, you can toss just about anything in this recipe and count on it being delicious!


We love Monster cookies around here almost as much as we love my favorite chocolate chip cookies and Reese’s Pieces peanut butter cookies.

If you’ve never had monster cookies, they are cookies with peanut butter, oats, chocolate chips, and M&M’s. They’re sort of a mash-up of peanut butter cookies, M&M cookies, and oatmeal cookies.

Monster cookie bars are an even easier bar version of those cookies!

They’re thick, chewy, just a little gooey, and definitely the perfect way to use up a little leftover Halloween candy. I can only imagine how good these monster cookie bars would be with chopped Snickers, Reese’s Cups or Twix.


If you’ve made my sugar cookie bars, you already know that cookie bars are even easier to make than individual cookies. And the extra fun part of this recipe is that you can change up the add-ins to whatever you have in your pantry!

Add-in options

In addition to semisweet or milk chocolate chips, monster cookie bars have a heaping cup of mix-ins.

So what can you include?

Like I mentioned above, classic M&M’s are traditional, so those are always a great option.

But don’t feel like you have to stop there! Some other ideas include:

  • Reese’s Pieces
  • Mini Reese’s Cups
  • Salted peanuts
  • Mini pretzels
  • Raisins
  • Chopped mini candy bars
  • Peanut M&M’s
  • Caramel M&M’s

Get the idea? You can use a cup of one add-in or mix together a variety of your favorites.

This is such a great way to use up leftover Halloween candy. After all, we all know that after a week or so, you get tired of finding random mini candy bars around the house.

What kind of peanut butter and oats should you use?

You will want to use creamy peanut butter for these cookie bars. You can use regular peanut butter or the no-stir natural peanut butter, but I don’t recommend using the natural peanut butter that they grind right in the store.

It tends to be more grainy and isn’t great for baking. You want a super smooth peanut butter for this recipe.

Also, make sure you use old-fashioned oats in these bars. Quick-cooking oats will not work in this recipe.

Making this recipe

Start by lining a 13×9-inch pan with foil. Make sure to leave about 1 inch of foil hanging over the sides so you can easily pull the monster cookie bars out of the pan later.

Spray the foil lightly with nonstick spray and set the pan aside.

Use a mixer to cream the butter, peanut butter, and sugars together on medium speed until light and fluffy. This will take about 3 minutes.

Add the vanilla and eggs and mix to combine.

Whisk together the flour, baking soda, salt, and oats. Add the dry ingredients to the butter mixture, then stir in your chocolate chips and add-ins.

Spread the batter into the prepared pan. If you’d like, you can add extra goodies such as M&M’s, pretzels, etc, onto the top; just make sure to press them lightly into the batter.

Bake the bars for about 30-35 minutes. Like chocolate chip blondies, I prefer to slightly under-bake these cookie bars than over-bake them.

I also like to sprinkle the tops of my monster cookie bars with some flaky sea salt right after they come out of the oven, but that’s totally optional!

Let the bars cool completely before removing them from the pan and cutting into squares.


Store monster cookie bars in an airtight container at room temperature for up to 3 days.

If you’d like to store them for longer, keep them in the freezer.

Place them in an airtight container with layers of parchment paper or wax paper between the layers. Let them thaw at room temperature, or take them out 1 or 2 at a time and microwave them for about 30 seconds to thaw and warm them up!


  • 1/2 cup unsalted butter, room temperature
  • 1 1/2 cups creamy peanut butter
  • 1 cup light brown sugar, packed
  • 1 cup granulated sugar
  • 2 teaspoons pure vanilla extract
  • 3 large eggs
  • 1 cup all-purpose flour
  • 2 teaspoons baking soda
  • 1 teaspoon kosher salt
  • 3 ¼ cups old fashioned oats
  • 1 cup semisweet or milk chocolate chips
  • 1 heaping cup of add-ins such as M&M’s, Reese’s Pieces, Mini Reese’s Cups, salted peanuts, mini pretzels, etc. (See notes)
  • Sea salt, optional


  1. Preheat oven to 350°F. Line a 9×13 inch baking pan with foil, leaving about 1 inch of the foil extending over the edges of the pan. The extra foil will create handles that you can use to lift the Monster Cookie Bars from the pan. Spray foil with nonstick cooking spray. Set pan aside.
  2. In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with an electric mixer, cream butter, peanut butter and sugars on medium speed for about 3 minutes. Add in vanilla and eggs and mix until combined.
  3. In a medium bowl, whisk together the flour, baking soda, salt and oats. Gradually add the dry ingredients into the butter mixture until thoroughly incorporated. Stir in chocolate chips and add-ins.
  4. Spoon batter into prepared pan and spread into an even layer. If desired, top batter with additional chocolate chips,  M&M’s or other add-ins, pressing them lightly into the batter. Bake in preheated oven for 30-35 minutes. If desired, sprinkle bars with flaky sea salt after removing from oven. Place pan on a cooling rack and cool completely before cutting into squares.


Monster Cookie Bars can be stored in an airtight container at room temperature for up to 3 days.

I sometimes like to use Hoody’s Peanut Butter Chocolate Mix from Costco in these bars since it has all of the mix-ins we love. 

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Nutrition Information

Yield 15

Serving Size 1 bar

Amount Per Serving

Calories 564Total Fat 28gSaturated Fat 11gTrans Fat 0gUnsaturated Fat 14gCholesterol 58mgSodium 477mgCarbohydrates 69gFiber 4gSugar 43gProtein 12g

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