Pre-heat oven to 350 degrees. Spray muffin tin with non-stick cooking spray and set aside.
In a medium bowl combine flour, graham cracker crumbs, baking powder, and salt. Set aside.
In a large bowl, cream the butter and brown sugar together until smooth. Add in the vanilla and eggs.
Add dry ingredients to butter/sugar mixture and mix until just combined (don't overmix). Take about ¾ of the dough for the base of the cookies (saving ¼ for the topping). This doens't have to be extact!
In the greased muffin tin, scoop about 1.5 tbsp dough into the muffin cup and press down. Place 2 pieces of the chocolate bar on top of the dough. Then dollop about ½ Tbsp marshmallow fluff on top of the chocolate.
Crumble the remaining dough that was saved over top the marshmallow- see picture (some of the marshmallow will still be visible- that's ok! You don't need to cover completely).
Bake 12-15 minutes or until golden brown. Allow cookies to cool completely and enjoy!