Line a 32cm legnth traybake tin with greaseproof paper.
In a large bowl stir together flour, sugar, cocoa and espresso powder.
Add the vanilla extract, milk, eggs and sunflower oil and beat together until smooth.
Slowly pour in the boiling water bit by bit, mixing in-between, until the mixture is smooth and glossy.
Pour into the baking tin and bake for 25-30 minutes until the cake has risen and a toothpick inserted into the middle comes out almost clean with only a few crumbs.
Leave to cool for 20 mins in the baking tin, then transfer to a cooling rack and leave to cool completely.
Meanwhile finely chop the dark chocolate into very small chunks and place in a bowl.
Heat the double cream in a saucepan set over a low flame. Stir continuously until the cream is piping hot and has almost reached boiling point.
Pour the cream over the chocolate and stir together until the chocolate has melted and the ganache is smooth and glossy. Cover and leave for 1 hour at room temperature to cool and thicken.
Use a palette knife or spoon to spread the ganache over the top of the cake.
Notes
Store the cake at room temperature, in an airtight container, for up to 3 days.