I've never had a chocolate cake sink, scratch or doctored, but I've had more than a few scratch vanilla cakes sink over the years!
I bake at 350 for most cakes, never tried 300. I wonder if that might be a part of your problem, starting your cake on too low a heat?
My doctored chocolate cake I use DH deep chocolate or dark chocolate mix, 3/4 cup self rising cake & pasty flour, a scant 3/4 cup superfine sugar, 4 large eggs, 1 cup sour cream, 1 tsp vanilla, 1/4 tsp almond extract (sometimes I'll omit the almond) a scant 1/3 cup oil, a heaping tablespoon Hersheys chocolate syrup, and 1 cup chocolate milk. I'll add more chocolate milk if the batter looks too thick. I never use water for the liquid in doctored mixes. I use buttermilk, milk, chocolate milk, or chocolate milk mixed with Kahula, or Baileys, or other liqueurs for adult cakes.
Hershey's Black Magic cake is one of my favourite chocolate scratch cakes. Pretty well all Hershey chocolate cakes are good though.