Preheat the oven to 180°C/gas mark 4. Line the bottoms of both sandwich tins with baking parchment and butter the sides.
Put the cocoa and 100g dark muscovado sugar into a bowl with a bit of space to spare, and pour in the boiling water. Whisk to mix, then set aside.
Cream the butter and caster sugar together, beating well until pale and fluffy; I find this easiest with a freestanding mixer, but by hand wouldn't kill you.
While this is going on - or as soon as you stop if you're mixing by hand - stir the flour, baking powder and bicarb 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 drop in one egg, quickly followed by a scoopful of flour mixture, then the second egg. Keep mixing and incorporate the rest of the dried ingredients for the cake, then finally mix and fold in the cocoa mixture, scraping its 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 five to ten minutes, before turning the cakes out to cool.
But as soon as the cakes are in the oven, get started on your frosting: Put the water, 30g muscovado sugar and 175g butter in a pan over a low heat to melt. 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 before whisking till smooth and glossy.
Leave 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 down, on a cake stand or plate, and spread with about a third of the frosting, then 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.
Recipe from Nigella: Kitchen by Nigella Lawson (published by Chatto & Windus).