Preheat your oven to 350°F. Line an 8x8-inch baking pan with parchment paper
In a large bowl, mix together the melted butter, brown sugar, and granulated sugar until smooth.
Slowly add in the egg, egg yolk, and vanilla extract. Mix until fully combined.
Next, add in the flour, baking soda, and salt just until everything is full mixed together. Do not overmix the batter.
Fold in one cup of the chocolate chips, saving the rest for the filling and topping.
Using a cookie scoop, add the cookie dough evenly over the graham cracker evenly. Press gently into an even layer. Not too hard. Save some cookie dough on the side, though, don't use it all.
Add marshmallows and chocolate chips like I'm doing in the video.
Add the remaining cookie dough over the marshmallows, leaving some marshmallows exposed, and add the chocolate chips on top.
Bake at 350 degrees for 25–30 minutes, or until the edges are golden brown and the center is just set.
Remove from the oven and let the bars cool in the pan for 20 minutes. While it's still warm, press pieces of a Hershey's chocolate bar on top.
Remove from the pan and cut the bars with a sharp knife. Enjoy!
