Preheat oven to 350 F. Place graham crackers in a single layer in the bottom of a lightly greased 9x13-inch baking dish.
In a large bowl, whisk together flour, sugar, cocoa powder, salt and baking powder. In a large measuring cup, melt butter. Mix 1/2 of the chocolate chips into the melted butter. Stir until the chocolate is fully melted.
Add eggs and butter mixture into the dry ingredients. Stir until a thick dough forms. Gently fold in the remaining chocolate chips.
Pour batter evenly over graham crackers. Bake for 30-35 minutes. Evenly top with mini marshmallows. Broil in the oven for 1-2 minutes, until marshmallows are lightly browned.