Although the frosting here is divine, for a less rich, but still wonderful cake, leave off the frosting and dust with powdered sugar.
For the frosting:
1/4 cup cold water
5 tbs. coconut butter (this is not the same as coconut oil)
1/4 cup dark brown sugar
1 1/2 tsp. instant espresso powder
1 1/2 tbs. unsweetened chocolate (minimum 70% cocoa solids), chopped
For the cake:
1 1/2 cups all-purpose flour
1 1/2 tsp. baking soda
1/2 tsp. fine sea salt
1 1/2 tsp. instant espresso powder
3/4 cup unsweetened cocoa powder
1 1/2 cups dark brown sugar
1 1/2 cup hot water, from a recently boiled kettle
6 tbs. coconut oil
1 1/2 tsp. apple cider vinegar or white wine vinegar
Heat the oven to 350°F, then start with the frosting. Put all the frosting ingredients except the chopped chocolate in a heavy-based saucepan and bring to a boil. Once everything is dissolved, turn off the heat and add the chocolate. Swirl the pan to melt the chocolate, then whisk into a dark, glossy frosting. Set aside to cool.
Line the bottom of a 8″ springform pan with parchment paper. Put the flour, baking soda, espresso powder and cocoa in a bowl and fork to mix. Combine the sugar, hot water, coconut oil and vinegar and stir to melt. Add to the dry ingredients, then pour into the prepared pan. Bake for 35 minutes, or until a tester comes out clean apart from a few crumbs. Transfer to a wire rack and leave to cool in the pan.
Back to the frosting, give it a good stir to check the consistency. It should be runny enough to cover the cake but thick enough to stay mostly on the top. Unmold the cooled cake and place on a serving plate. Pour the frosting over the top. At this point you can decorate the cake with edible rose petals, chopped pistachios, etc.. Leave to stand for 30 minutes before slicing.