Put 1 inch of water in a medium pot and bring to a simmer.

Place sugar, cream, chocolate and butter in a heatproof bowl and place over the water to gently warm the ingredients. The bowl should fit snuggly over the pot (alternatively, use a double boiler). Let ingredients melt and combine without stirring for 10 to 12 minutes. The sugar should be fully dissolved or the sauce will have a granular texture.

Stir in espresso or espresso powder until well combined. Allow to cool slightly before using, or keep in the fridge for up to 2 weeks and warm before using.