Preheat oven to 160°C. Brush a round 22 cm (base measurement) cake pan with melted butter. Line base and side with non-stick baking paper.
Place butter, chocolate, sugar, water, golden syrup and vanilla essence in a heavy-based saucepan. Stir over medium-low heat with a wooden spoon for 5 mins or until chocolate melts and mixture is smooth. Set aside for 20 mins to cool
Add eggs, 1 at a time, beating well after each addition. Sift combined flours over chocolate mixture and stir with a wooden spoon until well combined.
Pour mixture into pan and bake in preheated oven for 50-60 mins or until a skewer comes out almost clean. Stand cake for 20 mins before turning onto a wire rack to cool.