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.

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.