Preheat oven to 350 degrees. Line a baking sheet with parchment paper and spray paper with cooking spray.
In a large bowl, cream together the butter and sugar. Add in the eggs and vanilla and mix with hand mixer until combined. Sift in the flour, baking powder, and salt and mix to combine. Fold in the chocolate chips and marshmallows.
Pour the batter into the prepared pan and spread with rubber spatula or damp hands. Sprinkle on the crushed graham crackers.
Bake for 25-30 minutes until lightly golden brown. Cool on wire rack for 15 minutes before using parchment to lift out of pan. Cool completely before cutting.
Video
Notes
Make ahead and store in an airtight container in the refrigerator for up to a week.