HEAT oven to 350℉. Grease and flour a 9×12.5×2.5-inch baking pan.
CREAM butter, dark brown sugar and granulated sugar in a large bowl with an electric mixer (or with a paddle attachment). Beat on high speed until light and fluffy.
REDUCE speed to low and add vanilla; then add eggs, one at a time. Scrape down the bowl.
SIFT together cake flour, baking powder and salt. Add flour mixture to butter mixture and beat until just combined. Fold in chocolate chunks and graham crackers.
POUR batter into prepared pan. Sprinkle marshmallows over top.
BAKE for 30 minutes or until a toothpick inserted in the center comes out clean. Let stand until completely cool, cut into squares and serve.
