Okay, so it's not really a secret anymore if I share it here, is it? I'm not sure why I've been possessive of this recipe online before, but I've had a few people ask for it and my sister said to share it, so I'm sharing it.
My Favorite Great-Auntie's Chocolate Pie
2 C evaporated milk
1 T butter
1 t vanilla
3 egg yolks
1.5 C sugar
4 T flour
4.5 T Hershey's cocoa (remember – has to be Hersheys)
1/8 t salt
Combine the milk, butter, vanilla, and egg yolks together.
Combine the sugar, flour, cocoa, and salt together.
Sift the dry into the wet in a saucepan. Cook on medium high, heat until thick (can sometimes take a long time).
This recipe doesn't seem to make enough for a deep-dish pie, so I always triple the recipe and make two pies. You could always 1.5x the recipe too, or just use a regular pie dish, not a deep dish.
Also, I'm pretty sure the rationale behind just using egg yolks was so you could save the whites for meringue. Nobody in my family likes meringue, so I stopped making it a long time ago. Instead, when I triple the recipe, instead of using 9 egg yolks and tossing 9 egg whites, I just put in 5 whole eggs. Seems to work okay.
My Auntie never wrote this recipe down and was never quite sure how to tell someone else to do it – every time she tried guessing it came out different. She didn't measure, but just knew. Her pies were always perfect. Ours, however, never were. Eventually my sister perfected the proportions. Here it is! But I don't know what it is about the cocoa. I've tried several different kinds (Nestles and Girhardelli) and neither of them work in this recipe. Has to be Hersheys.