2) Increase the heat to medium and boil uncovered until the caster sugar turns a warm chestnut brown (about 180°C on a thermometer), about 5 to 7 minutes, gently swirling the pan to stir the mixture. Be careful the mixture is extremely hot. Watch the mixture very carefully at the end, as it will go from caramel to burnt very quickly. Turn off the heat.
3) Stand back to avoid splattering and slowly add the cream and vanilla. Don't worry - the cream will bubble violently and the caramel will solidify.
4) Simmer over low heat, stirring constantly, until the caramel dissolves and the sauce is smooth, about 2 minutes. Allow to cool to room temperature, at least 4 hours. It will thicken as it sits.