Line a 8x8 baking pan with foil, using two perpendicular sheets so there is an overhang for a "sling" for easy removal.
In a bowl, combine graham cracker crumbs and melted butter, mixing well until mixture resembles wet sand. Press evenly and firmly into bottom of prepared pan, and set aside.
In bowl of stand mixer, combine slightly softened butter with both sugars. Mix until mixture is light and fluffy, about 3-4 minutes. Add in the egg and vanilla extract, mixing just until well incorporated.
In a separate bowl, combine flour, baking soda, and salt. Whisk together well. Add dry ingredients to butter mixture, and mix just until incorporated; do not over-mix.
Using rubber spatula, fold chocolate chips in evenly into dough. Dough may be sticky. Spread dough evenly over graham cracker crust in pan.
Bake for about 15 minutes. Sprinkle with mini marshmallows and continue baking for about 10 more minutes or just until marshmallows start looking lightly toasted on top.
Cool on wire rack until reaches room temp. Melt Hershey's bar and drizzle over top of dessert (I use a Ziploc baggie with tiny bit of tip cut off.) Chill in fridge until ready to serve.
To serve, use foil sling to remove entire dessert onto cutting board. Using long serrated bread knife, firmly press straight down into dessert to cut into bars. Drizzle with melted chocolate, if desired.
Notes
For the graham cracker crumbs, you can either buy them pre-crushed or make your own by placing graham crackers in a ziplock bag and crushing them with a rolling pin.
Make sure to leave an overhang with the foil in the baking pan. This will make it easier to lift the whole slab of cookie out of the pan for easy cutting.
Take care not to overmix the dough when adding the dry ingredients. Overmix can make the bars tough and dense.
Feel free to customize your s'mores cookie bars with different types of chocolate. White chocolate, dark chocolate, or even flavored chocolate chips can all add a unique twist to the recipe.
Allow the bars to cool completely before cutting. This will help them hold their shape and make for neater, more uniform bars.
Drizzling the melted Hershey's bar over the bars not only adds a decorative touch but also an extra layer of chocolatey goodness. Feel free to use more or less chocolate, depending on your preference.
If you want to get creative, you can even add a layer of peanut butter or Nutella before adding the marshmallows.
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