Brush lid and inside of a non-stick metal 2L pudding basin with butter. Line the base with a disc of baking paper.

In a small saucepan, bring water to the simmer. Add cocoa powder and whisk for about 1 minute until mixture has thickened slightly. Set aside to cool.

In a medium sized bowl, mix together flour, bicarbonate soda and salt.

In a large bowl, whisk sugar and oil to blend. Whisk in eggs and vanilla extract. Add flour mixture alternating with buttermilk until all mixed together. Whisk in cocoa mixture.

Pour batter into basin and cover with lid. Place a small upturned heatproof saucer in the base of a large tall pot (or use a large steamer with lid) and put the basin on top of that. Pour enough boiling water to reach halfway up the sides of the basin. Bring water to simmer over high heat. Cover pot and reduce heat to low. Steam pudding without removing lid for 1 hour and 40 minutes, or until it springs back when gently pressed or a toothpick is inserted into centre of pudding comes out clean. Carefully lift basin from pot and set it on a wire rack. Let pudding cool for 10 minutes before turning out onto a plate to serve.