Monster Cookie Bars

If you're craving cookies but want to get a little more creative, especially around Halloween, this monster cookie bar recipe is just the recipe for you! It's made with a super simple peanut butter cookie base and filled with hearty oats, yummy chocolate chips, and an iconic Halloween candy–though you can really add whatever mix-ins your heart desires. Adding candy eyes to your finished cookie bars is a great way to make this a shareable spooky treat for your Halloween party!

For more cookie bar recipes, try Snickerdoodle Cookie Bars, Gooey Candy Bar Blondies, Homemade Twix Bars, and Homemade Samoa Bars.

Why you'll love these monster cookie bars

• Rich Peanut Butter Flavor: The cookie dough base is loaded with rich, smooth peanut butter for a super sweet, nutty flavor and chewy texture!
• Customizable Mix-Ins: There are zero rules when it comes to the mix-ins. Add your favorite candy, chocolate, nuts, sprinkles, or anything else your heart desires!
• Quick & Easy Recipe: Minus the chill time, these cookie bars come together in under an hour, making it a great recipe when you're on a time crunch or if you're making them a day ahead!

Ingredient Notes

• Peanut Butter: I recommend only using a thick, creamy peanut butter like Jif or Skippy. 
• Chocolate: Use a mixture of Reese's Cups and high quality milk, semi-sweet, or even dark chocolate chips.
• Oats: Use old fashioned oats for the richest flavor.
• Sugars: You'll need both granulated sugar and dark brown sugar.
• Unsalted Butter: Use high quality European-style butter for best overall flavor.
• Flour: Use high quality unbleached all-purpose flour best results.
• Eggs: Bring your egg to room temperature quickly by placing it in a bowl of warm water for 10 minutes.
• Vanilla: Add a high quality vanilla extract for optimal flavor.

See recipe card for full information on ingredients and individual quantities.

Step by Step Instructions

Here are step by step photos and instructions on how to make this monster cookie bar recipe! For the full ingredient list and method, see the recipe card at the end of this post.

STEP 1: Mix dry ingredients. In a medium or large mixing bowl, stir together the flour, cornstarch, baking powder, baking soda, and salt. Set aside.

STEP 2: Cream butter and peanut butter. In the bowl of a stand mixer fitted with a paddle attachment, cream together the butter and peanut butter on medium speed for 2-3 minutes. Add the sugars and continue beating for 2-3 more minutes until light & fluffy.

STEP 3: Add eggs and vanilla. Beat in the egg, egg yolk, and vanilla until fully combined.

STEP 4: Add dry ingredients. Add the dry ingredients and mix on low speed until just combined.

STEP 5: Fold in the mix-ins. Lastly, fold in the oats, Reeses pieces, and chocolate chips with a rubber spatula.

STEP 6: Press dough into pan. Press the dough into the pan and top with more Reese's pieces and chocolate chips.

STEP 7: Bake. Bake the monster cookie bars for about 22 minutes or until evenly golden. Allow to cool completely in the pan before removing. 

STEP 8: Cool & serve. Cut into 16 squares, top with candy eyes, and enjoy!

Expert Baking Tips

• Use a thick & creamy peanut butter. I recommend using a thick, creamy peanut butter like Jif or Skippy.
• Use a variety of chocolate! Use a variety of chocolate chip sizes, as well as a mixture of semi-sweet, bittersweet, and milk chocolate chips for visual appeal and balance of chocolate flavor.

All of the recipes on this blog are carefully developed with gram measurements so you can easily recreate them in your own kitchen with success. Volume measurements are extremely inaccurate and leave room for significant errors. Not all measuring cups are made equally, so your one cup of flour will be different from my one cup of flour. By providing precise measurements in grams (aside from minor ingredients, which are given in tsp/tbsp), you can make these recipes accurately and with less cleanup! All you need is this kitchen scale. 

If this still isn't enough to convince you, I have provided volume measurements in the recipe card. If you are interested in understanding the conversions, this is the best conversion chart.

But trust me, once you try baking in grams you'll never turn back!

Happy baking! x