Start of by lining the base of a deep 20cm square cake tin with non-stick (baking) paper. Pre-heat the oven to 170 degrees Celsius (or 150ºC fan-forced).
Place the caster sugar and cocoa powder in a bowl and beat in the milk.
Melt the butter and chocolate in a small saucepan (taking care not to overheat the mixture). Once the butter and chocolate mixture has melted, quickly remove from heat.
Add the chocolate and butter mixture into the sugar and cocoa mixture along with the sunflower oil.
Beat in the eggs until smooth, stir in the yoghurt and vanilla extract and mix in the flour and baking powder.
Pour the mixture into the prepared tin, cover with a slightly domed sheet of aluminium foil and bake for an hour.
Remove the foil in the last 15 minutes of baking. Once done, skewer the cake to check if it's cooked.
Remove the cake from the oven and set aside to cool in the tin. Cover with clingfilm (while still warm) to keep it moist.
For the chocolate coating, mix the cocoa powder and milk until you get a smooth consistency. Whisk in the boiling water, then stir in the chocolate until melted.
Next, whisk in the icing sugar until it is dissolved before pouring it into a deep bowl.
Cut the cake into equal squares (9 or 12 portions). Dunk each piece in the chocolate coating and fish out with two forks.
Roll in shredded/grated coconut (it does get a tad messy at this point, but persevere) and leave to set.