Place the sugar, butter and cream in a pan and bring to the boil, stirring. Reduce the heat and simmer gently for 4-5 minutes, until thickened and beginning to become syrupy, stirring occasionally to prevent the mixture from catching. Remove from the heat and leave to cool a little.
Meanwhile, place the chocolate in a heatproof bowl set over a pan of simmering water until melted. Whisk the chocolate into the sauce until smooth and well combined. This is ready to serve warm or transfer to a bowl and leave to cool completely, then cover with clingfilm and keep in the fridge until needed. Use warm or cold as required.
Cook Ahead: This will keep happily for up to 1 week covered with clingfilm in the fridge.