Start by making the ganache. Chop the chocolate finely and place in a heat-proof bowl. Heat the coconut milk until it is just coming up to the boil.
Pour the hot milk over the chocolate and set aside for two minutes then stir until melted and smooth. Cover the bowl and place in the fridge to set for about 2 hours while you make the cake.
Preheat the oven to 180C/350F/gas mark 4 and grease and line a 23x33cm/9x13in rectangular cake tin.
Whisk together the milk, vinegar, caster sugar, brown sugar, sunflower oil and vanilla extract in a jug.
In a large bowl, sift together the flour, cocoa powder, baking powder, bicarbonate of soda and salt.
Gradually whisk the wet ingredients into the dry until smooth; be careful not to over-mix or the cake can become tough.
Pour the batter into the prepared tin and spread level. Bake for 25-35 minutes until a skewer inserted into the centre comes out clean.
Leave the cake to cool in the tin. Once cool, carefully flip it out onto a wire rack.
By now the ganache should be thick but spreadable; if it is too runny still then return it to the fridge until it is firm, or pop it in the freezer for a bit to speed up the process. If it is too thick then gently warm it over a pan of hot water until it has softened.
Spread the ganache over the top of the cake and top with chocolate curls, sprinkles or fruit.
The cake will keep for up to five days in an airtight container at room temperature. If it is warm, store the cake in the fridge so that the ganache doesn't become too soft.