Place the apples, salt, and sugar in a saucepan. Add enough water to almost cover the apples. Place over high heat and bring to a boil. Reduce heat and simmer just until the apples are cooked, about 15 minutes.
Drain the apples and place in a clean saucepan. Puree the apples with a stick blender. Add the butter and heat the mixture over low heat to evaporate any excess liquid.