These Thick and Chewy Monster Cookie Bars are everything you love about Classic Monster Cookies baked into one easy, irresistible pan! They’re thick, soft, and loaded with peanut butter, oats, melty chocolate chips, and colorful M&M’s – the perfect mix of chewy centers and crisp edges!

Classic Monster Cookies

Monster Cookie Bars with Peanut Butter and Oats!

If you love a cookie bar that’s thick, soft, chewy, and absolutely loaded with chocolate, these Thick & Chewy Monster Cookie Bars are about to become your new go‑to dessert! They’re everything you love about Classic Monster Cookies – oats, peanut butter, chocolate chips, and M&M’s – baked into one easy pan that slices into perfect squares every single time.

This is the kind of recipe you make for bake sales, potlucks, after‑school snacks, or those nights when you just want something cozy and sweet without fussing with scooping individual cookies. One bowl, no chilling, no complicated steps… just a pan of chewy, chocolate‑packed goodness!

Classic Monster Cookies

Why this Recipe Works

The Texture is Thick, Chewy & Perfectly Soft – A blend of flour and quick oats gives these bars their signature Monster Cookie chew. The oats add heartiness while the flour keeps everything stable and sliceable.

Corn Syrup Keeps the Bars Soft for Days – Just a small amount of light corn syrup prevents the sugars from crystallizing, which means your bars stay chewy, moist, and bakery soft – even on day three.

Chocolate Chips + M&M’s Give the Best Mix of Melt & Crunch – Chocolate chips melt into gooey pockets while M&M’s add pops of color and a little crunch. It’s the perfect contrast in every bite.

Slight Under Baking Guarantees Chewy Centers – Pulling the bars when the middle still looks a touch soft is the secret to that th

Close‑up of monster cookie bars showing their gooey center and crisp golden edges

Ingredients You’ll Need

3/4 cup Butter (Softened) – Adds richness and helps create a soft, chewy texture.

3/4 cup Sugar – Adds sweetness and helps the bars bake up with lightly crisp edges.

1/2 cup Brown Sugar – Adds moisture, chewiness, and a deeper caramel flavor.

2 Eggs – Binds the dough and keeps the bars tender.

3 tbsp Light Corn Syrup – Keeps the bars soft and chewy, preventing crystallization.

2 tsp Vanilla – Enhances all the warm, sweet flavors.

3/4 cup Peanut Butter – Adds creaminess, structure, and classic monster cookie flavor.

1 tsp Baking Soda – Helps the bars rise slightly and stay soft.

1/2 tsp Salt – Balances sweetness and boosts flavor.

1 3/4 cups Flour – Provides structure so sthe bars hold their shape.

1 3/4 cups Quick Oats – Adds hearty, chewy texture – essential for monster cookies.

1 cup Chocolate Chips – Makes melty chocolate pockets throughout.

1 cup M&M’s – A colorful crunch and signature monster cookie flair.

Freshly baked monster cookie bars cut into squares with visible oats and melted chocolate

How to Make Thick & Chewy Monster Cookie Bars

Preheat oven to 350 degrees. Spray 9 x 13 pan with non-stick spray and set aside.

In a large bowl, beat together 3/4 cup softened butter, 3/4 cup sugar, and 1/2 cup brown sugar until smooth and creamy.

Add in 2 eggs, 3 tbsp light corn syrup, and 2 tsp vanilla. Mix until well combined.

Mix in 3/4 cup peanut butter, 1 tsp baking soda, and 1/2 tsp salt.

Add in 1 3/4 cups flour and 1 3/4 cups quick oats and mix until just combined.

Fold in 3/4 cup chocolate chips and 3/4 cup M&M’s.

The dough will be a little sticky, so it’s easiest to drop the dough in small globs then spread until smooth.

Sprinkle and press 1/4 cup chocolate chips and 1/4 cup M&M’s into the top of the batter.

Bake for 28-32 minutes, or until edges and top are lightly golden brown.

Let bars cool completely before cutting.

Save Chocolate Chips & M&M’s for the Top – Pressing extra mix-in’s on top before baking makes the bars prettier!

Don’t Over Bake – These bars will finish setting as they cool. Over baking is the primary reason Monster Cookie Bars lose their chew. Look for golden edges and a golden center – but not too golden.

Cool Completely Before Slicing – Warm bars will give you messy, broken squares. Cooled bars will give you clean, bakery style cuts.