The ingredients for a chocolate cake from scratch are butter, sugar, eggs, flour, cocoa powder, baking soda, salt, milk and frosting. You can make frosting from scratch using butter, milk and powdered sugar.
Exactly what goes into a chocolate cake and in what quantities varies depending on the recipe. One example of a simple, classic chocolate cake recipe from Taste of Home calls for 2/3 cup of softened butter, 1 and 2/3 cups of sugar, 3 eggs, 2 cups of all-purpose flour, 2/3 cup of baking cocoa, 1 teaspoon of salt, and 1 and 1/4 teaspoons baking soda, and 1/3 cups of milk.
To make the cake, first cream the butter and sugar. Next, add the eggs one at a time, beating well after each egg. In a separate bowl, mix the flour, cocoa, baking soda and salt, then alternate adding that and milk to the creamed mixture. Finally, pour the batter into a greased and floured cake pan. Bake at 350 degrees Fahrenheit for 35 to 40 minutes. When the cake is finished and cooled, cover in frosting.
To make a frosting, mix 3 cups of powdered sugar and 1/3 cup of butter, then add 1 and 1/2 teaspoons of vanilla and 1 tablespoon of milk. Add more milk as necessary to make the frosting smooth and spreadable. To make chocolate frosting, add 3 ounces of melted and cooled baking chocolate to the mixture before adding the milk.