Line a 12-section bun tray with paper cake cases. Put the butter, muscovado sugar, eggs, flour, cocoa powder and baking powder in a bowl and beat with a handheld electric whisk for about a minute until light and creamy. Divide the cake mixture evenly between the paper cases.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 20 minutes or until risen and just firm to the touch. Transfer to a wire rack.
Spread the frosting over the tops of the cakes while still warm.