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. 

Our 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.