For cake: Melt chocolate and butter in double boiler (or microwave).
In a medium mixer bowl, beat eggs until foamy; add sugar and beat well. Add other ingredients and the chocolate mixture – mix well.
Bake in 13x11-inch pan at 325 degrees for 35 minutes. A 13x9-inch pan works as well.
For topping: five minutes prior to the cake coming out of the oven, cook the stick of butter, evaporated milk, chocolate and sugar until blended. Remove pan from heat and beat in powdered sugar and vanilla.
As soon as the cake is removed from the oven, cover it with 6 ounces mini-marshmallows and pour on hot icing.