Preheat oven to 350 deg F and line a 8x8 inches square baking pan with parchment paper, leaving over hangs for easy removal. Line the bottom of pan with graham cracker sheets, saving a few for topping. Set it aside.
Take oil, eggs and powdered sugar in a mixing bowl and beat it with a hand beater till light and fluffy, this should take about 2 minutes.
Beat in vanilla next. Then sift flour, cocoa powder, salt and baking powder over wet ingredients and beat till the flour disappears into the batter.
Pour batter in the prepared pan and bake for 25-30 minutes. A toothpick inserted in the middle should come out with a few crumbs.
Take the pan out of oven and top the brownies with chocolate chips, marshmallows and crushed graham crackers. Return to the oven and broil, under the broiling element until the marshmallows turn golden.
Broiling should take between 1-3 minutes depending on the heat of your oven.
Allow the brownies to cool and set before cutting.