Line a 12-section bun tray with paper cake cases. Put the lightly salted butter, caster sugar, eggs, flour, cocoa powder, baking powder and orange rind in a bowl and beat with a hand-held 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 to cool.
Melt the chocolate (see