Here is a recipe for Chocolate Cake in her handwriting that I think may be the one he's talking about. Its clearly a well-loved recipe from the state of the card, so I think this must be it.
There is also another recipe that she used a lot that she got from a neighbor, who's name I don't remember. If anyone has that, please let me know and I will post it.
The White Mountain frosting on the card is not what I remember her using on chocolate cake. I remember her using chocolate fudge for frosting. Besides being delicious, she said it kept better in care packages. She would sometimes flavor the chocolate fudge with pure mint oil for variation.
One reader commented that the recipe was difficult to read, so I've transcribed it here:
2 cups sugar
1 cup shortening (she probably used Crisco or butter)
1 cup buttermilk (or 1 cup milk + 2 T vinegar)
2 tsp vanilla
1/2 cup cocoa
2-1/2 cups flour
2 tsp baking soda dissolved in 1 cup hot water with a pinch of salt added
Cream butter and sugar. Add eggs, beat well. Then, add buttermilk. vanilla, and the cocoa. Beat well. Add flour alternately with the hot water with soda/salt.
Bake at 350 degrees in 10" angel food cake pan. Takes about 50 - 60 minutes.
Doesn't even need frosting, but if you want some,
Grandma made fudge frosting with real peppermint oil.