- Pre-heat oven to 180°C/350°F/Gas Mark 4. Place the sponge mix in a bowl fitted with whisk beaters. On a slow speed gradually add the water and coffee essence, scrape down. Increase the speed to medium and whisk for a further 4 minutes.
- Pour into a greased lines tin 25cm x 35cm (10" x 14") and bake for 25-35 minutes until golden brown. Leave to cool.
- Beat the margarine, icing sugar and vanilla essence together. Spread over the cool sponge and dust with cocoa powder.