These monster cookie bars are everything you could want – soft, chewy, and filled with peanut butter, oats, chocolate chips and M&Ms. It’s an easy cookie bar recipe that’s faster than making a batch of cookies and perfect if you like extra gooey cookies. 
**This post was updated September 10, 2023 with a slightly updated recipe for chewier bars, new photos and recipe tips**
If you’re looking for an easy cookie recipe that’s big on flavor – then these monster cookie bars are just the thing. They’re loaded with peanut butter flavor, chewy oatmeal, and tons of chocolate candies. You can definitely taste the peanut butter, but it’s not too overpowering. So you end up with cookie bars that are deliciously chewy, with the perfect balance of salty and sweet.
Now I absolutely love monster cookies – but turning cookies into cookie bars makes them even better. Instead of forming the dough into balls, it gets pressed into a large baking dish. I actually think cookie bars are better than their cookie counterparts for a few key reasons:
- They’re chewier and gooier
- You can pack them with more M&Ms and chocolate chips
- There’s no need to chill the dough
- It’s faster and less messy than making cookies.
Monster Cookie Bar Recipe
This recipe is seriously easy & seriously delicious. Here’s the rundown (or feel free to scroll down to the recipe card if you don’t want the extra explanation/cookie science).
- Start by preheating the oven to 350F (180C). I like to line the pan with parchment paper so that there’s an overhang around the edges. You can also lightly grease the pan instead. Just be sure to do one or the other.
- Beat together ¾ cup butter, 1 cup brown sugar, ½ cup white sugar and ¾ cup peanut butter. Using a combination of brown sugar and white sugar makes the cookies extra chewy. Note that I highly, highly recommend using commercially prepared, smooth peanut butter. Anything labelled natural or where the oil sits on top can lead to inconsistent results because the amount of oil added to the batter will differ.
- Then mix in 2 large eggs and 1 tablespoon of vanilla extract. Your eggs should be at room temperature for easier mixing. At this stage – the mixture should look almost fluffy and creamy.
- Then carefully mix in the dry ingredients: 1 ½ cups flour, 1 teaspoon baking soda & ½ teaspoon salt. Be careful not to add too much flour or the bars can get dry instead of soft and chewy. The dough will be sticky, which is expected.
- Stir in 1 ½ cups old-fashioned oats – AKA large rolled oats. Quick oats will work, however, I find old-fashioned gives the best texture and chew-factor. Do not use instant oats (they behave more like flour) or steel cut oats (they’re too hard for baking).
- Stir in ¾ cup M&Ms and ¾ cup chocolate chips. It’s a lot, but that’s the benefit of making cookie bars. You can fill them with more delicious mix ins. With regular cookies if you add too many chocolate chips, nuts or candies the cookies can’t hold together, but with cookie bars that isn’t an issue.
- Then spoon the mixture into the prepared pan and smooth into an even layer. You can do this using an angled metal spatula or by pressing it down using your hands. If using your hand, I recommend lightly greasing them first.
- Bake in the middle rack of your oven preheated. The bars will bake for 30-35 minutes, or until the top looks set and slightly browned around the edges. The bake time really depends on you and how gooey you like your bars.
Pro Tip: Normally, I make sure my brownies and bars are fully cooled before slicing. This ensures that the bars are fully set and creates cleaner cuts. As you can tell by the photos, I sliced into my bars when they were still warm – they cooled for only about 45 minutes making them ooey, gooey and half falling apart. Delicious. But if you want clean cuts, wait for the bars to cool fully before slicing. 
Can I turn these monster cookie bars into monster cookies? This recipe was developed to be made into bars. If you want to make regular monster cookies, then try this recipe instead.
Can I freeze these cookie bars? Yes, they freeze well. Make sure the bars have fully cooled first. Then wrap tightly and freeze. Thaw in the fridge first, then bring them up to room temperature before enjoying.
Love these monster cookie bars? Try these other M&M recipes:
Monster Cookie Bars
Equipment
- 9x13 inch (23 x 33 cm) baking pan*
Ingredients
- ¾ cup unsalted butter (168 grams) softened
- 1 cup brown sugar (210 grams)
- ½ cup granulated sugar (100 grams)
- ¾ cup smooth peanut butter not natural
- 2 large eggs
- 1 tablespoon vanilla extract
- 1 ⅔ cups all-purpose flour (208 grams)
- 1 teaspoon baking soda
- ¼ teaspoon salt
- 1 ½ cups old-fashioned oats
- ¾ cup chocolate chips your favorite variety
- ¾ cup M&Ms
Instructions
- Preheat the oven to 350F degrees. Line a 9x13 inch pan with parchment paper so there's a 1 to 2-inch overhang on all sides, or lightly grease.
- In a large bowl beat the butter, brown sugar, granulated sugar and peanut butter until you no longer see clumps of butter or brown sugar.
- Mix in the eggs and vanilla to the butter mixture until smooth and creamy.
- Turn the mixer down to low speed and mix in the flour, baking soda & salt.
- Stir in the oats. I do this by hand.
- Turn off the mixer and stir in the chocolate chips and M&Ms.
- Spoon the dough into the prepared pan and smooth into an even layer using an angled spatula. Alternatively, press it down into an even layer with lightly greased fingers.
- Bake for about 30-35 minutes or until the top looks golden and the bars are starting to pull away from the edges of the pan. The bake time will vary depending on your oven and how gooey you like your bars.
- Cool the bars in the pan for at least 40 minutes - if you only cool the bars for 40 minutes they will be very gooey when you slice in. If you lined the pan with parchment, lift them out of the pan using the overhang and place on a cutting board. Then slice using a sharp knife.
Notes
- Butter: If using salted butter, you can omit the salt from the recipe.
- Peanut Butter: Be sure to use smooth, commercially prepared peanut butter. I do not recommend using anything labeled natural or where the oil sits on top. These peanut butters (while delicious) can lead to inconsistent results due to different amounts of oil being added.
- Oats: I recommend using large, rolled oats as opposed to quick oats. Quick oats will work but don't provide the same amount of texture. Make sure to use rolled oats - not steal cut oats. Instant oats do not provide enough texture and behave more like flour instead of oats.
- M&Ms: I prefer to use regular instead of peanut butter because peanut M&Ms make the bars harder to slice.
- Storage: Store cooled bars in an airtight container at room temperature for up to 4 days. Baked and cooled bars can be frozen for up to 2 months, then thaw in the fridge.
- Nutrition: Nutrition details provided are an estimate only and based on 1 bar, assuming the bar is sliced into 18 equal bars.
Nutrition
I think my daughter and I will be looking forward in doing this for the coming holiday season. She loves M&Ms.