Preheat oven to 350º F.
Line an 8x8 baking pan with parchment paper, making sure the paper doesn't come up over the sides of the pan. Set aside.
In a medium sized mixing bowl, add the snickerdoodle cookie mix, butter, and egg. Mix until it comes together in a thick cookie dough.
Press the cookie dough into the prepared pan in an even layer.
Bake for 14 minutes until cookies just turn golden brown.
Remove the cookies from the oven, and turn the oven to preheat on broil.
Add the chocolate chips to the tops of the cookies. Add the marshmallows on top of the chocolate layer.
Place the pan back in the oven under the broiler (on the top rack), and watch as the broiler toasts the marshmallows. This will only take 1-2 minutes.
Fully chill the bars before slicing. Cut the cookies into 16 squares.
Store leftover cookies in an airtight container in the refrigerator for up to a week, or in the freezer for up to 3 months.