Adapted from just the cake part of Martha Stewart's "Salted Caramel Six-Layer Chocolate Cake."
Ingredient amounts in parentheses are for high altitude baking, 5,000 feet or higher.
*Dutch-process cocoa is darker and less acidic than regular cocoa. It can be hard to find, but you can't substitute regular cocoa for Dutch-process without also fiddling with the leavening to allow for the different acidity. Hershey's has a "Special Dark" brand of cocoa that is a blend of regular and dark cocoa and will work fine for this recipe. I have recently been using a brand that Costco carries, Rodelle Gourmet Baking Cocoa, European Dutch Processed, and have had good luck with it. If Costco stops carrying it I'll go back to the Hershey's Special Dark.
**Buttermilk is a difficult ingredient to keep on hand, as you don't usually need a lot of it for a recipe and it doesn't keep well. I always have a can of buttermilk powder on hand and use that. The directions are on the container and it's very easy to use. For each 1/4 cup of buttermilk called for in the recipe you use 1 tablespoon of the powder and 1/4 cup of water. Mix the buttermilk powder in with the dry ingredients and the water in with the wet ones (obviously!). You can also do the lemon juice or white vinegar trick, by adding 1 tablespoon of either to 1 cup of milk and letting stand for 5 minutes before using. I always feel that doing it that way is cheating, but hey! Sally over at Sally's Baking Addiction does it that way, and she's the pickiest person around when it comes to baking. I'll stick with my buttermilk powder, though. Your call.