Preheat oven to 350°F. In a stand mixer, mix together the butter, sugar, cocoa powder and vanilla on medium speed. Next, add in one egg at a time and fully incorporate it before adding the next. Mix in the flour and salt last, until the batter is creamy and smooth. Pour the the batter into a greased 1-inch baking sheet and bake on 350°F for 20 minutes.
While the cookie bar base is baking, cut the marshmallows in half and set aside.
In a small mixing bowl, whisk together the butter, cocoa, evaporated milk, vanilla, powdered sugar, and salt for the frosting. Mix until it is smooth and creamy.
Remove the cookie sheet from the oven and place the marshmallow halves in rows on top. There should be 10 marshmallow halves per row, and about 20 rows total. Put the cookie bars back in the oven for 5-6 minutes to let the marshmallows puff.
Remove the cookie bars from the oven and generously drizzle the frosting over the marshmallows. Place the cookie sheet in the refrigerator for at least 30 minutes to set. Serve chilled in 2 to 4 square servings, and store the leftovers in the refrigerator. Enjoy!
Feel free to sprinkle crushed walnuts on top for a traditional rocky road cookie.