Pre-heat oven to 160oC or 315oF. Grease the tin and line with baking paper, the collar should extend 2cm above the top of the tin.
Place the butter, chocolate and coffee into a saucepan with 160ml (2/3cup) water and stir over a low heat, until melted. Remove from heat.
Sift the flours, bicarbonate of soda and cocoa into a large mixing bowl. Stir in the sugar and make a well in the centre. Add the combined egg, oil and buttermilk and the chocolate mixture, stirring with a wooden spoon until completely combined.
Pour mixture into the prepared tin and bake for 1 hour and 40 minutes or until a skewer poked into the cake comes out clean - it might be a little sticky. Leave the cake in the tin until cold.
This cake will keep in an air tight container in the fridge for up to 3 weeks or frozen for up to 2 months.
Recipe by Coloured Sugar at https://www.colouredsugar.com/chocolate-mud-cake/