1. Combine the brown sugar, butter and cream or milk in a saucepan. Place over heat, stirring until the sugar is dissolved; then boil without stirring 3 to 5 minutes, until the mixture forms a soft ball when a small amount is dropped into a cup of cold water, 238° F. Remove from the heat.
2. Beat until the mixture is cool and thickened.
3. Beat in the vanilla.