I've been tinkering with this recipe for years--the original, the "pure" version is in the Fannie Farmer Cookbook, but I prefer my version, which is creamier, more choclatey, and less sweet, unless you add the optional chocolate chips or m&ms.
You will need:
Unsweetened cocoa powder
Vegetable oil (I use rapeseed oil; corn oil is fine, too. Olive oil is not okay, since it has too much of its own flavor)
Chocolate chips or m&ms (optional)
If you can't get buttermilk, you can "sour" milk by adding a teaspoon or slightly more of plain white vinegar or lemon juice to milk and let it stand for about 15 minutes.
Preheat oven to 180ºC (that's 350º F). Grease (I used butter) a springform pan. Set aside. In a large mixing bowl put 235 grams of flour (that's 1 2/3 cups) plus 1/2 cup sugar and 1/2 cup unsweetened cocoa powder. Add 1 teaspoon baking soda and 1/2 teaspoon salt. Mix with a fork or whisk. Add 500 mil. buttermilk (which is about 17 fluid oz.) and 1/2 cup vegetable oil. Add two or more teaspoons of vanilla--American vanilla; I recommend McCormick's brand. Now mix all with an electric beater and pour into the greased springform pan. Use a scraper to get every drop in there. At this point, you can sprinkle chocolate chips over the top--the German ones, which usually have a touch of cinnamon, are nice, but any chocolate will do.
Bake for about 20 to 25 minutes. Poke with chopstick--if it is still very gooey leave in a little longer. But some enjoy a gooey cake, which will solidify after it cools. If you like it very dry, keep it in longer--30 minutes or longer. When the cake cools, it is good with whipped cream on top.
This is our standard birthday cake.