In a large saucepan, bring 8 cups water to a boil. Add plums; cover and boil for 30-45 seconds. Drain and immediately place plums in ice water. Drain and pat dry. When cool enough to handle, remove skins. Cut in half; remove pits.
In a small saucepan, bring sugar and 1 cup water to a boil. Cook and stir until sugar is dissolved. Add lemon juice and zest; set aside to cool.
Place the plums in a food processor; add sugar syrup. Cover and process until smooth, 2-3 minutes. Transfer puree to an 8-in.- square dish. Freeze until edges begin to firm, about 1 hour; stir. Freeze until firm, about 2 hours longer.
Just before serving, transfer to a food processor; cover and process until smooth, 2-3 minutes.