Preheat oven to 170 degrees C/150 degrees C fan-forced. Grease a 6cm deep, 20cm round springform pan. Line base and side with baking paper.
Place butter and 200gm chocolate in a large heatproof bowl. Pour water into a saucepan until one-third full. Bring to the boil over high heat. Reduce to low heat. Place bowl over saucepan (don't let bowl touch water). Using a metal spoon, stir mixture for 3 minutes or until smooth and combined. Cool slightly.