1 teaspoon cream of tartar
This procedural text is making plain taffy, so this is not a material that is needed. It will hopefully be the result of the procedure. The other options (continue heating to a slow boil and boil to 310) are steps in the procedure. They are things you must do, not materials you can have. The ingredient to make the taffy, 1 teaspoon cream of tartar, is a needed material.
