Preheat the oven to 300 degrees. In the bowl of an electric mixer, cream the sugar and shortening together. Add eggs and vanilla and beats 30 seconds. In a separate bowl, sift the flour, cocoa and salt together. Add to the mixture and beat until well combined. Pour the mixture into a greased and floured 9 by13-inch pan. Place in the oven and bake for 35 minutes.
Remove pan from the oven and let cool for a few minutes. Spread the marshmallows on top of the cake and return the pan to the oven for 5 more minutes. Remove the pan from the oven and cool for a few minutes. Evenly apply the frosting. Allow this to stand about 2 hours before cutting into squares.
Note: If the marshmallows are too hot, the frosting will sink right through. The frosting should be the top layer. It will darken once it hits the heat of the cake.
To make the frosting: In the bowl of an electric mixer, beat together the butter, cocoa, powdered sugar and vanilla. Slowly add the milk.