I made my boyfriend a chocolate cake for his birthday yesterday (today's his actual birthday, but he has to work so we did things yesterday). This one's really super chocolaty and rich, and I recommend using good ingredients (I use Ghirardelli's Dutch Process Cocoa) I also like to use my electric mixer, I normally mix and stir by hand, but for cakes you can get better results from going electric. My cake holder is only big enough for two layers, if you want a third layer just increased ingredients by half.
First, mix your dry ingredients: 2 cups of all purpose flour, 2 cups of granulated sugar, 1 cup of cocoa, 2 teaspoons baking soda, 1 teaspoon baking powder, and 1 teaspoon salt, beat on low until they're all mixed together.
Now add your wet ingredients: 3 eggs, 1 cup of buttermilk (I much recommend over regular milk), 1 cup of warm water, 1/3rd of a cup of olive oil (you can use vegetable if you like, but I prefer olive oil), and 2 teaspoons of vanilla extract. Beat on medium speed until completely smooth.
Grease and dust your pans with flour ... I like to use coconut oil, but you can use butter and that's okay. Get your clumps out as best you can.
Pour your batter evenly into your pans .. I like using a large spoon to scoop it out, that way I know I'm keeping them even. I like to bang my pans on my counter a couple times, you'll see a lot of air bubbles rising out, it helps your cakes rise a bit better.
Bake at 350 degrees for 35 minutes, then your cakes will need to cool for about 15 minutes still in their pans, then transfer from your pans to a wire rack and let cool for several hours. You can later wrap them in plastic and put them in the fridge, but I don't, I've never found that necessary.
Get started on your frosting, mix three sticks of soft butter (unsalted) with 8 ounces of cream cheese, blend until you've got a nice and fluffy cream.
Next add 1 and 1/2 cups of your cocoa and 1 tablespoon of vanilla extract. Keep beating until it's all combined together, it might get sort of difficult as your mixture gets super creamy, you'll get a sort of paste-like texture.
You'll next need to add 8 cups of powdered sugar (a whole bag), but just one cup at a time. Beat at slow to medium speed, and keep adding cups of sugar as it's blending in. Then mix in 1/4 cup of milk.
Your frosting will be really creamy now, and very thick .. I like to leave it out just a little bit so it'll be soft and easy to spread.
After your layers have cooled, you'll need to slice off the domes so they're flat. Use a sharp knife to gently cut in a circle around the dome, then use a large serrated knife to slice the tops off, cutting around in that circle you made. Your tops should peel right off, and those make a lovely treat for anyone who may be helping you.
Spread a thick layer of frosting on your bottom layer to hold them together, and then place your top layer on upside down, kind of like if you're making a giant Oreo cookie.
Spread frosting over the top and sides of your cake, and if you don't have a scraper then use a bread knife to smooth the edges.
Enjoy!