Preheat the oven t 180°c / Gas 4. Grease a 20cm springform cake tin and set aside.
In a bowl as the margarine and caster and with an electric mixer whisk until smooth and creamy.
In a bowl as the flour, cocoa and baking powder. In a large glass as the eggs and lightly whisk.
Add a third of the egg to the mixture and stir until combined, then add a 3 of the dry ingredients and mix together. Repeat until all mixed together.
Add the milk and orange essence and stir together. Add the chocolate chips, mix together and then spoon into the cake tin. Bake in the oven for 30 - 40mins until a skewer inserted in the middle comes out dry and if you gentle press the cake in the middle it springs back.
Remove from the oven cool in the tin for 5 mins, then transfer to a wire rack to cool complete.
For the topping
Melt the chocolate with the double cream and butter in a heatproof bowl over a pan of simmering water, then leave to cool.
Spread the chocolate thickly over the top, decorate and then place in the fridge for 30mins to set.