Preheat the oven to 350°F / 180°C. Line two 8in / 20cm round cake pans with parchment paper.
Whisk 50g of cocoa powder and 100g of dark muscovado sugar with 250ml of boiling water and set aside.
Beat 125g of softened unsalted butter with 150g of superfine sugar until pale and fluffy. In another bowl, mix 225g of all-purpose flour, ½ teaspoon of baking powder, and ½ teaspoon of baking soda.
Beat 2 teaspoons of vanilla extract into the butter, then one of 2 large eggs, a spoonful of the flour mixture, and the other egg. Mix in the remaining flour mixture, then fold in the cocoa mixture.
Divide between the pans and bake for about 30 minutes, until a cake tester comes out clean. Cool on a wire rack.
Meanwhile, for the frosting, heat 125ml of water, 30g of dark muscovado sugar, and 175g of cubed unsalted butter over low heat. When it starts to bubble, take it off the heat and stir in 300g of finely chopped dark chocolate.
Leave the frosting for about 1 hour, whisking now and then, until thick enough to spread. Spread a third between the cooled layers and the rest over the top and sides.
