Preheat oven to 350°F and line a couple cookie sheets with parchment paper.
Add the graham crackers to a food processor or in a zip top bag. Pulse until resembles crumbs or smash into crumbs in the bag. Measure out 1 cup of graham cracker crumbs and set aside.
In bowl of a stand mixer or with hand mixer and large bowl, cream together the butter, brown sugar and sugar.
Add the egg and vanilla extract and beat until light and fluffy, scraping down the sides of the bowl.
Add the baking powder, baking soda, salt, all purpose flour and graham cracker crumbs. Mix on low speed just until combined. Dough will look a little dry and crumbly.
Add the cold water and mix on low just until the dough comes together.
Using a small cookie scoop or 1 tablespoon, scoop dough and place 2 inches apart on prepared cookie sheet.
Bake cookies for 7 minutes.
Immediately place marshmallows cut side down on top of each cookie.
Place cookies back in oven and bake for another 2 minutes.
Immediately press a Hershey's bar rectangle on top of the marshmallows.
Let cookies cool for 3 minutes on pan and then remove to wire rack to cool completely.