Preheat the oven to 180C/350F/Gas 4. Line the bottoms of two 20cm/8in sandwich tins with baking parchment and butter the sides.
Put the cocoa and the dark muscovado sugar into a large bowl and pour in the boiling water. Whisk to mix, then set aside.
Cream the butter and caster sugar together in a separate bowl, beating well until pale and fluffy; I find this easiest with a freestanding mixer, but by hand wouldn’t kill you.
Stir the flour, baking powder and bicarbonate of soda together in another bowl and set aside for a moment.
Dribble the vanilla extract into the creamed butter and sugar – mixing all the while – then crack in one egg, quickly followed by a scoop of the flour mixture, then the second egg. Keep mixing after each addition.
Incorporate the rest of the flour mixture little by little, then finally mix and fold in the cocoa mixture, scraping the bowl well with a spatula.
Divide this fabulously chocolatey batter between the two prepared tins and put in the oven for about 30 minutes, or until a cake tester comes out clean. Take the tins out and leave them on a wire rack for 5-10 minutes, then turn the cakes out and set aside to cool.
As soon as the cakes are in the oven, get started on your frosting. Put the water, muscovado sugar and butter into a pan over a low heat until melted.
When this mixture begins to bubble, take the pan off the heat and add the chopped chocolate, swirling the pan so that all the chocolate is hit with heat, then leave for a minute to melt. Once melted, whisk until smooth and glossy.
Set the frosting aside for about one hour, whisking now and again – when you’re passing the pan – by which time the cakes will be cooled, and ready for the frosting.
Set one of the cooled cakes, with its top side facing downwards, onto a cake stand or plate, and spread with about one-third of the frosting. Top that with the second cake, regular way up, and spread the remaining frosting over the top and sides, swirling away with your spatula. You can go for a smooth look, but I never do and probably couldn’t.