Mix Butter and Sugar: Beat the butter with an electric mixer for 30 seconds at medium speed. Add the brown and white sugar, and beat for another 2 minutes until fluffy and light.
Add Egg and Vanilla: Beat in the egg, scrape the bowl, then add the yolk and mix. Add vanilla extract and mix on low.
Combine Dry Ingredients: Add flour, baking powder, baking soda, salt, and graham cracker crumbs. Mix on low until the dough forms.
Add Mix-Ins: Stir in the chocolate chips, chopped graham crackers, and 3/4 cup mini marshmallows. You can chop the marshmallows roughly to prevent them from expanding too much.
Shape Dough Balls: Scoop out a heaping tablespoon of dough for each cookie, forming about 26 balls. Roll each between your hands to make them smooth.
Top and Chill: Place a piece of Hershey's chocolate on top of each cookie. Chill the cookies in the fridge for at least 1 hour, or freeze until solid (about 15-30 minutes).