Preheat the oven to 180°C and insert a baking sheet.
Boil all icing ingredients except chocolate with 60ml water until dissolved.
Remove from heat, add chopped chocolate, let sit for one minute, and whisk until glossy.
Line the bottom of a 20cm leakproof springform tin with baking parchment.
Mix flour, bicarbonate of soda, salt, espresso powder, and cocoa in a bowl.
Melt sugar, water, coconut oil, and vinegar together and stir into the dry ingredients.
Pour batter into the tin and bake for 30 to 35 minutes until a tester comes out clean.
Cool the cake completely in the tin on a wire rack before removing.
Stir the icing and pour it over the cooled cake.
Decorate with rose petals and pistachios and let set for 30 minutes before slicing.