Name: Monster Cookie Bars
Prep time: 15 min
Cook time: 25 min
Total time: 40 min
Cuisine: American
Ingredients: 1/2 cup unsalted butter, melted; 3/4 cup creamy peanut butter (Jif or Skippy recommended); 3/4 cup brown sugar, packed; 1/2 cup granulated sugar; 2 large eggs + 1 egg yolk; 2 teaspoons vanilla extract; 1 1/2 cups old-fashioned rolled oats; 1 cup all-purpose flour; 1 teaspoon baking soda; 1/2 teaspoon salt; 1 cup M&M’s; 1/2 cup chocolate chips

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Looking for a crowd-pleasing treat that’s ridiculously easy to make? These Monster Cookie Bars deliver all the beloved flavors of classic monster cookies in a simple bar form. Just one bowl, no chilling time required.

I’ve perfected this recipe to get that ideal chewy texture using melted butter and an extra egg yolk. These easy monster cookie bars are perfect for bake sales, family gatherings, or whenever you need something sweet fast.

Why You’ll Love This Monster Cookie Bar Recipe

Here’s what makes this recipe absolutely unbeatable:

One-Bowl Wonder: Everything mixes together in a single bowl, meaning minimal cleanup and maximum convenience
No Chilling Required: From ingredients to oven in under 15 minutes – no waiting around for dough to firm up
Perfectly Soft & Chewy: Our secret combination of melted butter and an extra egg yolk creates that coveted bakery-style texture
Classic Monster Cookie Flavor: The perfect balance of peanut butter richness, hearty oats, and sweet chocolate surprises
Completely Foolproof: With step-by-step photos and a helpful video, success is guaranteed even for beginning bakers
Feeds a Crowd: One 9×13 pan makes 24 generous bars – perfect for potlucks, bake sales, or large families

The Key Ingredients You’ll Need

Let me walk you through each ingredient and why it matters for creating the perfect chewy monster cookie bars:

Butter (1/2 cup, melted): We use melted butter instead of softened butter because it creates a denser, chewier texture. The melted butter coats the flour proteins differently, preventing too much gluten development.

Peanut Butter (3/4 cup): Stick with creamy, commercial brands like Jif or Skippy for best results. Natural peanut butter contains too much oil separation and can make the bars greasy. The peanut butter acts as both a flavor base and a binding agent.

Brown Sugar (3/4 cup, packed): Brown sugar’s molasses content adds moisture and creates that signature chewy texture. Don’t substitute with white sugar – you’ll lose the depth of flavor.

Granulated Sugar (1/2 cup): This balances the brown sugar and helps create the perfect sweetness level.

Eggs (2 large eggs plus 1 extra yolk): Here’s our secret weapon! That extra egg yolk adds richness and helps bind everything together for ultra-chewy results.

Vanilla Extract (2 teaspoons): Pure vanilla extract enhances all the other flavors without overpowering.

Old-Fashioned Oats (1 1/2 cups): This is crucial – use old-fashioned rolled oats, NOT quick-cooking or steel-cut oats. The texture and baking time depend on this specific type.

All-Purpose Flour (1 cup): Provides structure without making the bars too cake-like.

Baking Soda (1 teaspoon): Creates just enough lift for a tender crumb.

Salt (1/2 teaspoon): Enhances all the flavors and balances the sweetness.

Mix-ins (1 cup M&M’s + 1/2 cup chocolate chips): The classic combination, but feel free to get creative with our variations below!

How to Make Monster Cookie Bars

Step 1: Prep Your Pan and Oven

Preheat your oven to 350°F (175°C). Line a 9×13 inch baking pan with parchment paper, leaving some overhang for easy removal. This prevents sticking and makes cleanup a breeze.

Step 2: Mix the Wet Ingredients

In a large mixing bowl, whisk together the melted butter, peanut butter, brown sugar, and granulated sugar until smooth and well combined. The mixture should be glossy and uniform. Add the eggs, extra egg yolk, and vanilla extract, whisking until fully incorporated.

Step 3: Add the Dry Ingredients

Add the oats, flour, baking soda, and salt to the wet mixture. Using a wooden spoon or spatula, gently fold everything together until just combined. Don’t overmix – you want to see a few streaks of flour, which will disappear during baking.

Step 4: Fold in the Mix-ins

Gently fold in the M&M’s and chocolate chips, reserving a handful of each to press on top for that bakery-perfect look.

Step 5: Press and Bake

Transfer the dough to your prepared pan and press it evenly into the corners. Don’t pack it too tightly – just spread it evenly. Press the reserved M&M’s and chocolate chips on top for visual appeal.

Bake for 20-25 minutes, until the edges are set and lightly golden but the center still looks slightly soft. This is key for achieving that perfect chewy texture!

Baker’s Tips for Perfect Cookie Bars

After making these easy monster cookie bars countless times, here are my professional tips for guaranteed success:

Don’t Over-mix the Dough: Once you add the flour, mix just until the ingredients are combined. Overmixing develops too much gluten, resulting in tough, cake-like bars instead of chewy ones.

The “Slightly Under-baked” Trick: For the ultimate chewy texture, pull the bars from the oven when the center still looks a little soft and underdone. They’ll continue cooking from residual heat and firm up as they cool.

Let Them Cool Completely: This is the hardest but most important step! Cutting warm bars will cause them to fall apart and lose their shape. Allow at least 2 hours of cooling time for clean, professional-looking squares.

Press Extra Toppings on Top: For that bakery-perfect appearance, gently press a few extra M&M’s and chocolate chips on top of the dough before baking. This creates beautiful, colorful tops that photograph wonderfully.

Use Room Temperature Eggs: Cold eggs can cause the melted butter to seize up. Let your eggs sit at room temperature for 30 minutes before mixing, or place them in warm water for 5 minutes.

Variations & Substitutions

One of the best things about this monster cookie bars recipe is how easily you can customize it:

Holiday Variations:

Use red and green M&M’s for Christmas
Orange and black for Halloween
Pastel colors for Easter
School colors for team events

Mix-in Alternatives:

Reese’s Pieces instead of M&M’s for extra peanut butter flavor
Butterscotch chips for a caramel-like sweetness
Chopped pretzels for salty crunch
Mini marshmallows for s’mores vibes
Crushed Oreo cookies for cookies and cream flavor

Dietary Modifications: Gluten-Free Monster Cookie Bars: Simply substitute the all-purpose flour with a 1:1 gluten-free baking blend (I recommend King Arthur or Bob’s Red Mill) and ensure your oats are certified gluten-free. The texture remains perfectly chewy!

Dairy-Free Option: Use vegan butter and dairy-free chocolate chips. The texture stays remarkably similar.

Storing, Freezing, and Making Ahead (FAQ)

How do I store monster cookie bars?

Store completely cooled bars in an airtight container at room temperature for up to 4-5 days. Layer parchment paper between stacked bars to prevent sticking. They actually taste even better on day two as the flavors meld together!

Can you freeze monster cookie bars?

Absolutely! These freeze beautifully for up to 3 months. Cool completely, cut into squares, and wrap individually in plastic wrap before placing in a freezer-safe container. Thaw at room temperature for about an hour before serving.

Can I make the dough ahead of time?

Yes! You can prepare the dough up to 2 days in advance. Cover the bowl tightly with plastic wrap and refrigerate. Let it come to room temperature for about 30 minutes before pressing into the pan and baking. You may need to add 2-3 extra minutes to the baking time.

Why are my bars falling apart?

This usually happens when the bars haven’t cooled completely before cutting. The binding ingredients need time to set up properly. Always wait at least 2 hours before cutting, even if it’s tempting to dig in sooner!

Can I double this recipe?

Definitely! Double all ingredients and use two 9×13 pans, or bake in a large half-sheet pan. Keep the same baking temperature but check for doneness at 25 minutes, as larger pans may need slightly longer baking time.

Monster Cookie Bars