Preheat oven to 350 degrees F. Grease a 9x13'' pan with non-stick cooking spray.
In a large mixing bowl, pour oil over cocoa powder and mix.
Add softened butter and mix until smooth. Add sugar and mix for 1 minute. Add eggs, one at a time, mixing after each addition. Add vanilla and mix.
Add flour and salt and stir to combine.
Pour batter into prepared pan. Bake for 25-30 minutes or until a toothpick inserted into the center comes out clean.
Remove brownies from the oven and sprinkle the marshmallows evenly on top. Return to the oven for 2-3 minutes or until the marshmallows are puffed. Remove from oven and allow them to cool completely.
Once brownies are cool, make the frosting.
For the frosting:
Add melted butter, cocoa powdered, evaporated milk, vanilla, and 2 cups of powdered sugar to a mixing bowl and beat with electric beaters until smooth.
Add more powdered sugar, a cup at a time, mixing well, until you reach your desired frosting consistency. I usually use about 3 1/4 cups powdered sugar.
Spread frosting over the cooled brownies and cut into squares. I've found a plastic knife works best to cut them, or if you make them 1 day in advance they are much easier to cut.
Tip for Cutting Frosted Brownies: Use a plastic knife! The frosting and marshmallows will cut smoother and easier, without making a sticky mess, if you use a plastic knife to cut the brownies. Rocky Road Brownies: Mix in 1 cup nuts to the brownie batter, or add nuts at the same time as the marshmallows.Make Ahead Instructions: I almost like these brownies even better the second day. Something about the way the flavors blend together and settle after several hours makes them even more irresistible. I also feel like they're a little less "rich" on day two, and they are easier to cut. But, it may be hard to wait that long, so feel free to dig in right away. The frosting can be made up to 2 weeks in advance, stored in an airtight container in the fridge. Freezing Instructions: The brownies can be frozen frosted or unfrosted, covered well, for up to 3 months. Allow to come to room temperature before serving.