If you love the flavors of s'mores but don't want to fuss with a campfire, these chocolate chip marshmallow cookie bars are the answer. All the gooey, toasty s'mores flavor baked into an easy 9x13 pan of cookie bars, no skewers or open flame required.

These bars take the same easy, no-mixer approach as my chocolate chip cookie bars and add graham cracker crumbs and mini marshmallows for a s'mores twist. If you've made my stuffed s'mores cookies before and loved that combination of chocolate, marshmallow, and graham, you'll find a lot to like here too, just in bar form instead of individual cookies.

The 9x13 pan recipe makes this one especially convenient for serving large crowds.  There's no scooping individual cookie dough balls, no need for an electric mixer, and no waiting around for butter to come to room temperature.

Melt the butter, mix everything by hand, and pour it all into one pan. Whether it's a backyard barbecue, a potluck, or just a weeknight treat, these bars deliver classic s'mores flavor with a fraction of the effort, and they're easy to cut into squares and share with a crowd.

Ingredients Needed:

• All purpose flour, salt: the base of your cookie bar and needs to be measured out accurately for the best results in texture. If you don't have a scale, fluff the flour then spoon it into your measuring cup and level it off. 
• Baking powder: adds to the rise of cookie bar
• Espresso powder: this is an optional flavor enhancer. It's not a strong coffee taste in the cookie bar but works well to cut the sweetness of the sugar. You can also use 1 teaspoon ground cinnamon if you prefer or omit it all together. 
• Graham cracker crumbs: for the s'mores flavor. Crush graham crackers in a food processor to get fine crumbs. It takes about 20-22 square crackers to 1 cup of crumbs.
• Eggs: I've used 2 eggs here plus an extra yolk to keep the bars moist and tender.
• Vanilla extract: Always use pure vanilla extract (not imitation vanilla) for the best flavor.
• Unsalted butter: Melted butter is added for  flavor and softness. You can also substitute it for brown butter if you want a deeper nutty flavor. Read my tutorial for how to make brown butter for guidance. 
• Brown sugar: Brown sugar to keeps baked goods softer than granulated white sugar. You can also use a mix of dark and light brown sugar or just dark sugar. Be sure the sugar is completely soft so it mixes well with the melted butter. If you have hardened sugar you can re-soften using the method outlined in my How to Soften Brown Sugar post. 
• Semi-sweet chocolate chips: Though semi-sweet is a personal favorite, you can also use milk chocolate chips, dark chocolate chips or a combination of any if you prefer. 
• Mini marshmallows: I used vegan marshmallows (Dandies) here so you can be sure that they will work too. Freeze them for an hour ahead of mixing so they hold their shape better in the oven. Whether you use vegan or regular marshmallows is up to you as long as they are mini marshmallows! 

How to Make Chocolate Chip Marshmallow Cookie Bars

Here's a quick overview but be sure to read the recipe card completely before starting to bake.

Prepare your baking pan by buttering a 9x13 pan and lining it with parchment, leaving overhang on the two long sides for easy lifting later. I like to hold the overhangs in place with metal binder clips. Preheat the oven to 350°F.

Whisk together the flour, graham cracker crumbs, salt, baking powder, and espresso powder if using, then set aside. In a separate bowl, whisk the eggs, egg yolk, and vanilla.

Melt the butter gently either on the stove or in the microwave, then stir in the brown sugar until smooth and let it cool enough so it does not react with the eggs. Whisk in the egg mixture, then fold in the dry ingredients along with the chocolate chips and marshmallows, mixing just until the flour disappears. Overmixing will toughen the bars, so stop as soon as it's combined.

Spread the batter into the pan, pressing down any marshmallows poking up so they don't scorch in the oven. 

Bake for 20 to 22 minutes, until golden and a toothpick inserted in the center comes out clean. Keep an eye on the pan in the oven in the second half of the baking time.

If it looks like it's browning too much but the inside is still raw, loosely cover the top with a large piece of aluminum foil. Once done, let the bars cool completely in the pan before lifting them out using the parchment overhang. Slice into 24 squares once cooled.

Storage

Chocolate Chip Marshmallow Cookie Bars will stay fresh stored in an airtight container at room temperature for up to 5-7 days. 

Tips for Success

• Spoon your flour into the measuring cup and level it off rather than scooping straight from the bag. This one habit makes the biggest difference in whether your bars turn out soft or dense.
• Give the melted butter and sugar a few minutes to cool before adding the eggs, so they blend in smoothly instead of scrambling.
• Fold in the flour just until it disappears. A few extra stirs is all it takes to turn soft bars into tough ones.
• Push down the marshmallows into the batter before baking to prevent scorching.
• Keep an eye on the oven in the last few minutes. If the top is browning faster than the center is setting, a loose sheet of foil will slow it down without underbaking the middle.
• Resist cutting into the bars while they're still warm. A full cool-down is what keeps the squares intact instead of crumbling apart.