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If you love peanut butter, chocolate, and oats, these Monster Cookie Bars are for you. Made with a box of yellow cake mix {like this Salted Caramel Cookie Bar}, this easy recipe creates thick, chewy bars with consistent results—no scooping and no extra steps required.
A Quick Look at the Recipe
- Recipe Name: Monster Cookie Bars
- Prep Time: 10 minutes
- Bake Time: 30 minutes
- Servings: 12 servings
- Time Saving “Practically Homemade” Ingredient: Yellow cake mix
Table of Contents
Key Ingredients
- Yellow Cake Mix: Using a box of yellow cake mix is a total time-saver here. It takes care of the flour, leavening, and base flavor, so you can skip measuring out multiple dry ingredients. One of the many reasons it shows up again and again in my best recipes using yellow cake mix.
- Oatmeal: The oats give these bars that classic chewy texture. Quick oats work best because they blend into the dough smoothly without being too chunky. I use them the same way in these Cake Mix Oatmeal Cookies.
- Peanut Butter: Any creamy peanut butter works great here. If you want a stronger peanut butter taste, feel free to add an extra tablespoon or two.
See recipe card for full information on ingredients and quantities.
How to make Monster Cookie Bars with a Cake Mix
Step 1: Preheat your oven to 350°F. Spray a 9×9 baking pan with nonstick cooking spray, or line it with parchment paper. In a large bowl, combine the yellow cake mix and brown sugar. Whisk them together to break up any lumps.
Step 2: Add the melted butter, peanut butter, egg, milk, and vanilla extract. Use an electric mixer to combine everything until smooth. Add the quick oats and mix until evenly distributed. The dough will be thick.
Step 3: Fold in the mini chocolate chips and M&M’s until they’re spread throughout the dough.
Step 4: Spread the dough into your prepared pan in an even layer.
Step 5: Bake for 30-35 minutes, or until the top is golden brown and the edges are set.
Step 6: Let the bars cool completely in the pan before slicing into squares.
Expert Tips
- Don’t overbake these bars. Cookie bars made with cake mix might look slightly underdone in the center when you pull them out, but they’ll continue to firm up as they cool. If you bake them until they’re completely set in the oven, they’ll end up dry and crumbly instead of chewy.
- Let them cool completely before cutting. This is important! If you try to slice these bars while they’re still warm, they’ll fall apart. Give them at least an hour to cool in the pan, and you’ll get clean, neat squares.
- Use room temperature ingredients. Your egg and milk should be at room temperature so everything mixes together smoothly. Cold ingredients can make the melted butter seize up and create a lumpy batter.
- Switch up the mix-ins. You can easily customize these bars with different candies or chips. Try peanut butter chips, Reese’s Pieces, or even butterscotch chips. And for a chocolate chip version, try my Chocolate Chip Cookie Bars!
Storage and Freezing
Store: Keep leftover monster cookie bars in an airtight container at room temperature or in the refrigerator for up to 4 days. Refrigeration is optional but not required.
Freeze: Let them cool completely, then wrap individual bars in plastic wrap and place them in a freezer-safe bag or container. Freeze for up to 3 months. When you’re ready to eat one, just let it thaw at room temperature for about 30 minutes, or pop it in the microwave for 15-20 seconds
Monster Cookie Bars FAQs
Yes! If you use a 9×13 pan, your bars will be thinner and will bake faster – check them around 20-25 minutes. If you want thicker bars, use an 8×8 pan and add about 5-10 minutes to the baking time.
You can, but the texture will be different. Regular oats are thicker and chewier, so your bars might end up a bit more rustic and less cohesive. If you only have regular oats, try pulsing them a few times in a food processor to break them down before adding them to the dough.
Do you love bar cookies? Try these recipe…
Did you try this recipe?
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Monster Cookie Oatmeal Bars
Ingredients
- 1 box yellow cake mix, {13.25 ounces}
- ½ cup brown sugar, {packed}
- ½ cup butter, {melted}
- ½ cup peanut butter
- 1 large egg
- ¼ cup milk
- 1 teaspoon vanilla extract
- 1 ½ cups quick oats
- ½ cup mini chocolate chips
- ½ cup M&M's
Instructions
- Preheat the oven to 350°..
- In a large bowl, combine the yellow cake mix and brown sugar, whisk the ingredients together to remove any lumps. Add the melted butter, peanut butter, egg, milk and vanilla to the dry ingredients and mix using an electric mixer. Add the quick oats and mix until evenly distributed. Finally, add the chocolate chips and M&M's to the cookie dough and mix.
- Prepare a 9×9 baking pan by spraying it liberally with nonstick cooking spray {or line with parchment paper}. Add the cookie dough to the prepared pan and spread out into an even layer.
- Bake for 30-35 minutes or until the top is golden brown. Remove from the oven and let the cookie bars cool completely in the pan before slicing into squares. Serve and enjoy. Store leftovers in an airtight container at room temperature for up to 4 days.
Notes
Nutrition
Nutrition information is automatically calculated, so should only be used as an approximation.
These were so decadent and easy to make!
A top article to read. Thanks a lot for sharing this. Fine work. Keep it up.