Preheat your oven to 350 degrees. Spray a 9 by 13-inch pan with cooking spray and set aside.In a large bowl, mix together crushed graham crackers, melted butter, and sugar. Press into the bottom of the pan.In a separate bowl, mix flour, salt, and baking powder; set aside.Beat together granulated sugar and room temperature butter until creamy. Add the egg and vanilla, mixing until just combined. Slowly add the flour mixture until fully combined.Carefully spread the cookie dough mixture on top of the graham cracker crust.Evenly spread mini marshmallows and chocolate chips over the cookie dough. Pour sweetened condensed milk evenly over the top.Bake for 22 to 27 minutes or until the edges are golden brown. (Closer to 27 minutes may be required.)Allow the bars to cool overnight before cutting, or indulge with a fork for an immediate treat.