Marshmallow Syrup is an all-natural substitute for light corn syrup. In order to keep the sugar in the marshmallows from crystallizing, you need to add an “invert” sugar, which stabilizes the sugar crystals. Light corn syrup is an invert sugar, but you can easily make an invert syrup by boiling water, cane sugar, and cream of tartar. (Recipe is in the book.) Be sure to follow the directions and allow the syrup to sit in the pan to cool for 15 to 30 minutes before putting it in a jar or using it in a recipe.
The syrup will be extremely thick when at room temperature. You can bring it back to pouring consistency by heating it in a microwave for 1 1/2 to 2 minutes, or by putting the jar (if it’s heatproof) in a pan of simmering water until the syrup softens. It’s faster in the microwave.
Stir the sugar, water, salt, and cream of tartar until all of the sugar has been moistened.
When the mixture starts boiling, place a cover on the pan for 3 minutes.
Then remove the lid and insert a candy thermometer into the pan. Once the lid is removed, do not stir the syrup.
Allow the syrup to cool for 15 to 30 minutes, and then ladle it into clean jars. Cover immediately. You can also use the syrup at this point to make marshmallows or fluff.