Monster Cookie Bars {Easy One Bowl Recipe}

The cooling temperatures this week have gotten me pretty nostalgic for popping on the oven and baking up some holiday cookies from my childhood. Monster bars definitely fit the bill!

Think peanut butter-oat bars chock-full of chocolate chips and colorful m&ms. With minimal prep and maximum appeal, they're a Christmas dessert you'll want to make again and again.

Why You Will Love This Recipe

• Ultimate Cookie Mashup: They are sweet, salty, easy to make, and hard to resist. This delicious confection is a hybrid of your favorite oatmeal cookies, peanut butter cookies, and Chocolate Chip Cookie all rolled into one. Need I say more?
• Easy One-Bowl Wonder: Made in one bowl with simple ingredients, there is no rolling out dozens of cookies or waiting for batches to cool.
• Perfect for Leftover Candy: Got some leftover Halloween candy or Christmas treats? Maybe use them and turn these into the perfect cookie exchange recipe!
• Soft, Chewy, and Irresistible: Oats and Peanut Butter make these bars soft and chewy, while the M&Ms add a fun, colorful crunch in every bite.
• Freezer-Friendly: You can make a batch ahead of time and freeze them for later. Just wrap each bar in plastic wrap, pop them in an airtight container, and you'll have a quick, delicious treat whenever you want.

Ingredients and Notes

Below is an overview of what you will need to make these Monster Bars with notes for potential substitutions.

Eggs: You'll need 2 whole eggs. They help bind the ingredients and add moisture to the dough.

Flour: I used all-purpose flour in this recipe, but these cookie bars also work well with good quality gluten-free flour.

Brown Sugar: I typically use light brown sugar for a balanced flavor and moisture level. Dark brown sugar has a slightly stronger molasses flavor and higher moisture content.

Old-Fashioned Oats: Stick with old-fashioned oats for this recipe. Quick oats cook too quickly and can give your bars a doughy consistency.

Peanut Butter: I recommend using a creamy peanut butter for a smooth texture. Crunchy peanut butter works too if you like, but I avoid all-natural varieties as they can separate during baking. Make sure to stir the jar well.

Unsalted Butter: Soften to room temperature on the counter for about 1 hour but do not melt. Salted butter can work if you leave out the ½ teaspoon salt.

Cookie Bars Add-In Ideas

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 Reese’s Pieces
• Chopped candy bars like Snickers or Twix
• Pretzels
• Shredded Coconut
• Sprinkles or Sanding Sugar
• Cereal or Rice Krispies
• Heath or Toffee Bits
• Christmas or Valentine’s Day Candies

Holiday Monster Cookie Ideas

• Halloween: Use leftover Halloween candy bars, and for an extra spooky touch, add edible candy eyes after baking to make these bars truly “monster” themed.
• Christmas Monster Cookies: Use red and green M&Ms and/or add red and green sprinkles for a festive holiday treat.
• Valentine's Day: Use red, pink, and white M&Ms and/or sugar hearts to give your bars a romantic twist.
• Easter or Spring: Use pastel-colored M&Ms or mini robin’s eggs to create a bright and cheerful version of these cookie bars.

Tips for Perfect Monster Bars

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. You may cut the recipe in half recipe for an 8×8. However, other sizes can produce an overly thick, chewy bar. It may not bake evenly.
• Use parchment paper cut wider than the longer sides of the pan. It acts like handles and the whole bars are easy to lift out.
• Cream Butter and Sugar. If your butter is at room temperature, the mixture should be smooth. If you notice any bits or chunks of butter, let it warm for a few minutes, then mix again until fully combined. (left photo above)
• Be careful not to over-mix, as this can make the bars dense. (right photo above)
• Check for Doneness. To ensure your bars are fully cooked, check that the edges are golden brown. A toothpick inserted in the center should come out clean or with just a few moist crumbs. Overbaking will make the bars dry.
• Cool Before Cutting: Make sure to let the bars cool completely before cutting them into squares. I know it's tempting to dig in right away, but letting them cool prevents the chocolate chips and M&Ms from smearing and keeps the bars looking neat and tidy.