1.Turn on the oven to 180°C/350°F and let it heat up. Spray a deep 20cm (8-inch) round cake pan with cooking oil; line the base with a circle of baking paper, use a strip of baking paper to line the side of pan.
2.Put the butter, vanilla extract, sugar, eggs, sifted flour and cocoa, and the water into a large bowl; beat with an electric mixer on low speed until the ingredients are just mixed together. Turn up the speed to medium; beat the mixture for about 3 minutes or until it is smooth and pale in colour.
3.Put the mixture into the pan; use a spatula to spread it evenly over the base. Put the pan in the oven and bake about 1 hour. Take the pan out of the oven; leave it for 5 minutes. Turn the cake from the pan onto a wire rack, top-side up; leave it to cool.
4.Meanwhile, make the chocolate icing. Put the chocolate and butter in a small heatproof bowl. Half-fill a small saucepan with water; bring to the boil. Turn down the heat and let the water simmer. Put the bowl of chocolate mixture on top of the saucepan; stir the mixture until it is smooth. Add a little of the sifted icing sugar and some of the water; stir until mixture is smooth. Keep adding the sifted icing sugar and the water, a little bit at a time, stirring, until the icing is spreadable.
5.When the cake is cold, spread the chocolate icing over the top with the back of a spoon or a metal spatula.
Store cake in an airtight container for up to three days.Note