When (nearly) all of the sugar has melted and liquid is amber (caramel) colored, add the butter and stir until it has completely melted. CAUTION: hot mixture will bubble up when you add the butter.

Remove from heat and stir in the cream. Again CAUTION, the mixture will foam up considerably and possibly splatter. Keep stirring until caramel sauce is smooth. Stir in vanilla and salt if you wish.

To remove any undissolved crystal lumps, pour sauce through a wire sieve into a clean, heatproof container. Caramel sauce will thicken as it cools. Store in refrigerator for up to two weeks. Gently heat caramel sauce in microwave before serving.