Mix espresso powder or instant coffee with 240 ml of hot water . Stir until the coffee has dissolved. In a pan stir together cocoa powder, chocolate chunks, sugar and coffee.
Cook over medium heat, stirring, until the chocolate has melted. Bring mixture to a boil, then reduce heat to low and allow to simmer for 10 minute or until the sauce has reduced, stirring frequently,
Remove pan from heat then stir in vanilla extract. Leave to cool for 30 minutes before using. Store leftover sauce in an airtight container in the fridge for up to 2 weeks.