hi, your friend’s grandmother had a few secrets! :D I’d ask your friend what the cake was like first: was it a layer cake with the frosting in the middle and outside, or a loaf (like a pound cake)?
This looks to be a standard butter cake to me, which could work either way, but the way the ingredients are mixed could be different. For a lighter layer cake, you’d use the creaming method, and for a pound cake you’d use Rose’s reverse creaming method like most of the recipes in Cake Bible or Heavenly Cakes…
For pastry flour it is usually 2 parts AP to one part cake flour by volume or weight. If you don’t have either one, your best bet is to go with 1 3/4 cups bread flour and sift it with 1/4 cup cornstarch, then mix with the salt and leavening. That’s your dry mixture.
The water should be mixed with the chocolate—the easiest way is to bring it to the boil and pour it over the chocolate, then cover and let it stand util it melts, then whisk smooth. Let this cool, then mix with the sour cream and eggs. That’s your liquid mixture.
From here for standard creaming, you’d beat your butter and sugar, then alternately add your dry and liquid mixtures (3 parts dry alt with 2 parts wet) until it’s very well-blended.
For reverse creaming, add your butter and sugar to your dry mixture with about a third of the liquid. Mix very well, then add the rest of the liquids in 2 parts, beating for at least 2 minutes.
I’say this would bake in either a large loaf pan or 2 8-inch pans for the time given? Either way, stick a toothpick in to make sure it’s done. Good luck!
For extra reference, look up the Domingo Cake in Cake Bible, it is similar in that it uses sour cream, but cocoa instead of unsweetened chocolate.
PS I’m sorry it looks like I was too late for this morning! hope your cake turned out okay.