Preheat the oven to 180C/160C Fan/Gas 4 and pop in a baking sheet at the same time.
For the icing, put all of the icing ingredients except the chopped chocolate into a heavy-based saucepan and add 4 tablespoons cold water. Bring to the boil, making sure everything’s dissolved. Then turn off the heat but leave the pan on the hob. Quickly add the finely chopped chocolate and swirl the pan so that it is all underwater, so to speak. Leave for a scant minute, then whisk until you have a darkly glossy icing, and leave to cool. I find this takes exactly the amount of time the cake takes to make, cook and cool. But do give the icing a stir with a spatula every now and again.
Line the bottom of your springform cake tin (you will need a good, leakproof one as this is a very wet batter) with baking parchment.
Put the flour, bicarbonate of soda, salt, instant espresso and cocoa powder in a bowl and fork to mix.
Mix together the sugar, water, coconut oil and vinegar until the coconut oil has melted, and stir into the dry ingredients. Pour into the prepared tin and bake for 35 minutes. Though do check at the 30-minute mark to see if it is already done. When it’s ready, the cake will be coming away from the edges of the tin and a cake tester will come out clean, apart from a few crumbs. This is a fudgy cake and you don’t want to overdo it.
Once the cake is cooked, transfer the tin to a wire rack and let the cake cool in its tin. Once completely cool remove from the tin.
Turn to your icing, and give it a good stir with a spatula to check it is at the right consistency. It needs to be runny enough to cover the cake, but thick enough to stay (mostly) on the top. So pour over the cake, and use a spatula to ease the icing to the edges, if needed. If you wish to decorate, now is the time to do it. In which case, sprinkle joyously with rose petals and chopped pistachios or anything else that your heart desires; otherwise, leave it gleaming darkly and, indeed, sumptuously. Leave to stand for 30 minutes for the icing to set before slicing into the cake.