This one is near and dear to my heart. This is my mother's recipe - it has to be well over 50 years old. The cake is incredibly moist and the icing turns into a fudgy layer on top. Really, really good!
Turn the oven on to 350 degrees. Grease and flour a 9x13 inch pan.
Mix together flour, sugar, soda, salt and cocoa. Then, add buttermilk, oil, boiling water, vanilla and eggs. Beat thoroughly and pour into the greased and floured pan. (If the batter seems a little thin, you are right on track! :-) Bake 35-45 minutes, testing with a toothpick to determine doneness.
Start the icing 5 minutes before the cake is done. Combine sugar, corn syrup, milk and butter in a saucepan and boil 2 minutes (no longer) then add the chocolate chips, vanilla, and nuts. Mix until the chocolate chips are melted.
Sprinkle the miniature marshmallows all over the top of the baked cake as soon as it is removed from the oven, then drizzle the hot icing mixture over the marshmallows.