Heat oven to 160C/ fan 140C/ gas 3 and grease a 20cm round cake tin
Tip the butter, syrup and vanilla into a heatproof jug, add 300 ml boiling water and briefly whisk with a fork to combineSift the flour, sugar, cocoa and bicarbonate of soda into a large bowl, then in tip in the butter mixture. Stir until you have a smooth batter, then scrape into prepared tin. bake for 50 – 55 mins until the cake is well risen and springy to touch. Cool in tin for around 15 mins before turning out to cool on a wire rack.
To make glaze, put chocolate and syrup into a small glass bowl with 3 tbsp of water, and microwave for 30secs on high until melted and smooth. leave to cool, then pour over the cake before serving