Preheat oven to 350 degrees. Grease a square 8x8 or 9x9 pan with non-stick spray. Mix graham cracker crust ingredients together in a small bowl. Pour into greased pan and press into a level crust. Bake in preheated oven for 8-10 minutes. Remove from oven and keep oven on.
In a small saucepan, add 1/2 cup sugar, butter, and oil. Turn heat to medium and cook until butter is completely melted. Turn off and mix together. Let cool slightly.
In a medium bowl, whisk flour, cocoa powder, baking powder, espresso powder, and salt all together until combined. Set aside. In the bowl of a stand mixer with a paddle attachment, mix eggs, yolk, and 1 cup of sugar together until combined. With the mixer on low, very slowly pour melted butter mixture into egg mixture. Turn mixer off and add flour mixture and chocolate chips. Mix on low just until combined.
Pour mixture into baked graham cracker crust and bake in preheated oven for 25-30 minutes, depending on the size of the pan. Remove when toothpick comes out clean when inserted in the center.
Arrange halved marshmallows into one even layer on top of the baked brownies. Place back in preheated oven until the marshmallows turn toasty brown, about 5 minutes. Remove from oven and let cool for 1 hour before attempting to cut them. Any sooner and they could fall apart!