In a large bowl, mix together the sugar, flour, cocoa, baking powder, and salt.
Mix until blended.
In another bowl, mix the oil, eggs, and vanilla.
Pour both bowls together and mix until all ingredients are incorporated.
Fold in the mini marshmallows.
Pour brownie batter into a greased 9x13 baking pan.
Bake at 350 degrees for 20 to 25 minutes or until a toothpick comes out without any batter sticking to it.
Once the brownies are finished, remove from oven and let them cool.
To create the sundaes, cut the brownies into serving size bars, top with a scoop of ice cream, crushed graham crackers, and a drizzle of chocolate syrup. Enjoy!