Line a 12-section muffin tray with paper muffin cases. Put the cocoa powder in a heatproof bowl and whisk in the boiling water. Leave to cool.
Beat together the butter and muscovado sugar in a separate bowl until pale and creamy. Gradually beat in the eggs. Sift the flour into the bowl and stir in, then add the cocoa mixture and stir in. Divide the cake mixture between the paper cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 25 minutes or until risen and just firm. Leave to cool in the tin for 10 minutes, then transfer to a wire rack to cool completely.
Put the cream, icing sugar and water in a small saucepan and bring just to the boil. Remove from the heat and stir in 200 g (7 oz) of the chocolate. Leave to cool, stirring frequently until smooth and glossy.
Peel away the paper cases and spoon a little of the chocolate mixture over each cake, swirling it slightly down the sides using a small palette knife.
Melt the remaining chocolate (see