Preheat oven at 180 degrees C.
In a bowl or jug mix cocoa powder, instant coffee if using and boiling hot water. Stir and set aside.
Mix flour, baking soda, baking powder and salt and set aside.
Using a stand mixer or electric hand whisk or wooden spoon cream sugar and unsalted butter or margarine until pale in colour about 3 - 5 minutes depending on tool used.
Mix in egg one at a time and add vanilla essence.
Add a third of the flour mixture and combine, then half of the cocoa mixture. Repeat the procedure and finish with the last third of the flour mixture.
Mix until just incorporated.
Prepare cake tins by greasing with a little butter or margarine then lightly dusting with flour or lining with some parchment paper.
Divide the cake batter evenly between prepared three 7 inch round tins or two 8 inch round tins.
Level the batter with the back of a spoon and bake in the pre heated oven for 25 to 30 minutes or until when the cake is gently pressed at the centre it springs back.
Allow the cakes to cool completely before frosting.
For the buttercream, in a mixer or using a whisk, cream unsalted butter until pale in colour then add icing sugar, cocoa powder and salt.
Mix until creamy, to lighten add a tablespoon or two of milk.
Once cakes are cooled, remove from tins, level any domed cakes with a serrated knife and frost the cake with the chocolate buttercream.
Additionally, you can fill the layers with dark chocolate ganache to take it to the next level.