Add all the liquid ingredients to a bowl or jug and mix.
Into a different bowl, add all the dry ingredients except the chocolate. Mix until combined.
Now, pour the liquid ingredient into the dry ingredient and mix until just combined.
Roughly chop the chocolate and fold the chunks into the batter.
Grease a rectangular cake pan and line it with baking paper to ensure the cake can easily be removed after baking. Then, transfer the cake batter into the cake pan.
Bake the cake at 170°C fan for 1 hour 10 minutes. To check that the cake is done, pierce it with a cake needle/skewer/chop stick - if it comes out clean (minus the melted chocolate) then the cake is done.
Cool down the cake completely before adding the icing.
For the icing, add roughly chopped chocolate and icing sugar to a small bowl and place the bowl onto a pot that contains boiling water. Alternatively, add the small bowl to a microwave to melt the chocolate.Either way, stir regularly to ensure the chocolate melts evenly.
Once the chocolate has fully melted, pour the icing over the cooled cake.
You can serve the cake right away or wait until the icing has set. The cake stores well in the fridge for up to 4 days.