Preheat the oven to 180°C (356°F) and grease and line one 22 cm cake tin or two 18cm cake tins.
In a bowl, mix together the flour, cocoa, salt, baking powder and baking soda. Set aside.
In another bowl, whisk together the sourdough starter, eggs, vanilla, oil, milk and sugars.
Pour the the wet mixture into the flour mixture and fold gently for a minute before pouring over the hot coffee.
Mix briskly together but don't over mix.
Pour into the prepared cake tin and bake for 35-40 minutes until a skewer inserted into the cake comes out clean. If you are using two smaller cake tins, check them after 25-30 minutes.
Chocolate Buttercream
Beat the butter until light and creamy.
Carefully add in the icing sugar and cocoa (powder will puff up so take care!).
Once incorporated, beat on high for 5 minutes and add in 2-3 tablespoons of milk or cream as needed until it is light, creamy and spreadable.
Chocolate Ganache
Heat the cream until just simmering. Take it off the heat and add the chocolate chips. Leave it to sit for 5 minutes, then stir until it is glossy and the chocolate has melted.
Let it cool down until it has thickened a bit before pouring on the cake.
Notes
The calorie count for this recipe is for one serving of sourdough chocolate cake. It does not include the frosting or ganache topping.