Preheat oven to 180 degrees C (350 degrees F). Line two 18 cm (7 inch) cake pans with circles of baking paper. Grease very well inside the pans.
In a large mixing bowl combine sifted flour, sifted cocoa powder, baking soda, baking powder, cinnamon (optional) and sugar. Mix with a hand whisk to blend the ingredients.
In a separate bowl combine eggs, vanilla extract, olive oil and buttermilk. Whisk well until the mixture becomes smooth and uniform.
Pour the wet ingredients into the dry ones. Whisk gently, until combined.
At the end gradually pour the coffee (or water). Continue whisking carefully, until smooth. The batter should not be thick, neither too thin. If you see that the water is too much and will make the batter liquid, don’t use the whole quantity.
Transfer the batter into the prepared pans dividing it in relatively equal parts.
Bake for 40-50 minutes or until a toothpick inserted in the centre of each cake comes out clean.
Let the cakes cool for 5-10 minutes and then remove them from the pans: carefully run a knife around the edges, then hold the pan with kitchen gloves and invert on top of a wire rack or wooden board.
In a large mixing bowl combine butter, powdered sugar and cocoa powder. Mix them with a rubber spatula. Add vanilla extract and then start pouring the milk little by little, while beating with an electric mixer at low speed. When the milk is absorbed and the mixture becomes relatively smooth, turn the mixer to the highest speed and beat for 2-3 minutes or until fluffy.
Little by little add the melted chocolate. Continue beating with the mixer at high speed until the frosting becomes creamy and uniform.
Using a long serrated knife cut each cake horizontally into two equal layers. Place one layer on a serving plate and cover it generously with some frosting. Place another layer. Continue the process until you use all the layers. Leave about 1/3 of the frosting to cover the top and the sides + about 2 tbs for decoration.
You can decorate the cake as you like. I made truffles by mixing some frosting with carob powder and pumpkin seeds powder. Part of the truffles are covered with carob powder. Added some chocolate ball sprinkles as well.
If you don’t have buttermilk, use the same quantity of milk with 1 tbs vinegar.
You can use up to 20 cm (8 inch) cake pans for the same quantity of ingredients, just beware that the cake will have slightly lower height. For a 21-23 cm (8,5 -9 inch) cake pan, increase the ingredients with about 50%. Double the ingredients for a cake pan 24-26 cm (9,5-10 inch).