Preheat oven to 170°C (150°C fan-forced). Grease and line a 19cm diameter cake tin.

In a medium saucepan, combine water, sugar, butter and combined sifted cocoa powder and soda. Stir over heat, without boiling, until sugar fully dissolves. Bring to boil then reduce heat. Simmer, uncovered for 5 minutes. Transfer mixture to large bowl. Cool to room temperature.

Add flour and egg to cooled mixture. Beat with electric mixer until mixture is smooth and changed to a paler colour. Pour mixture into prepared dish.

Bake about 1 hour in the middle of oven. Check with a toothpick or skewer if cooked all the way through before removing from oven. Stand cake in baking dish 10 minutes before turning onto wire rack. Turn cake top-side up to cool.