Preheat oven to 160 C/ 325 F . Butter and flour a 6 inch round pan . Line the bottom with parchment (optional).
Whisk together all the dry ingredients - flour , sugar , cocoa , salt and baking soda-in a large bowl .
Add all the wet ingredients - oil, vanilla , vinegar ,water - and whisk until smooth.
Pour into prepared pan and bake at 160 C/ 325 F for 45- 50 minutes until top springs back when touched and a skewer inserted in the centre comes out dry.
Cool in pan for 5 minutes and remove from pan to cool completely on wire rack.
While the cake is baking , chop your chocolate and heat the water for ganache. Dissolve the coffee in the water if using.
Melt the chocolate in a double boiler or microwave.
Add the hot water (or coffee) little by little to the melted chocolate , mixing well after each addition.
It will seem clumpy but keep adding all the water and mixing until smooth. Rest at room temperature until thick enough to use.
Slice the cake horizontally into 3 layers and fill with the ganache