In a medium saucepan, combine the sugar, salt, cream of tartar, and water.

Heat over medium high heat, stirring often with a rubber spatula to encourage even cooking.

When it comes to a boil reduce heat to medium and continue stirring often.

When the color turns an even medium amber color remove from heat and add the heavy cream, quickly mixing until fully combined.

Then, mix in the butter and vanilla.

Transfer to a glass jar to cool, stir in a pinch or two of flaky sea salt, then chill in the fridge overnight or until ready to use.