Combine the instant espresso powder and hot water, stirring until fully dissolved; set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and cream cheese together until smooth. Add the egg, espresso liquid and vanilla and continue beating until smooth. Add the cake mix and beat until incorporated. Cover and chill dough in the refrigerator for at least 2 hours so that it's easier to work with.
Preheat oven to 350ºF and line an 8-inch square pan with parchment paper. Flatten half of the brownie dough evenly into the bottom of the pan. Place a layer of the chocolate bars on top of the dough, reserving the rest for later. Spread the marshmallow creme evenly on top of the chocolate bars.
Flatten pieces of the last half of the brownie dough** and place on top of the marshmallow layer. Bake in the oven for 22-24 minutes, or until the brownie looks set and is dry to the touch. Pan will still be jiggly due to the marshmallow creme layer. Allow to cool completely***, about 2 hours.
Just before serving, preheat the broiler. Top the brownies with mini marshmallows and broken graham crackers. Place under the broiler for 45-60 seconds, or until marshmallows are golden brown and puffy. Be sure to keep your eye on it the WHOLE time so that it doesn't burn. Mine took about 50 seconds.
Turn the oven off, take the brownies out and immediately top with the leftover Hershey bar and chocolate chips. Return to the now OFF oven and allow the residual heat to slightly melt the chocolate for about 20-30 seconds. Remove from the oven and serve!