Pre-heat your oven to 350°F, and line two baking sheets with parchment paper.
In the bowl of a stand mixer, beat the butter and sugar together until combined.
Add the egg and vanilla extract, and mix until combined.
Add the graham cracker crumbs, flour, baking powder and salt. Stir until just combined. Try to avoid over mixing.
Fold in the mini marshmallows and chocolate chunks.
Divide the dough into 12 even sized balls, and place them on the parchment lined baking sheets about 4 inches apart. The cookies will spread quite a bit.
Bake for 9-11 minutes, until the cookies are puffed and look dry on top.
Let the cookies cool on the baking sheet for about 10 minutes before transferring them to a wire rack to cool completely.