1. Pit the plums, slice them, and put them in a medium-sized saucepan with 1/4 cup (60ml) water. Cover and cook over medium heat, lifting the lid and stirring occasionally, until the plums are soft and completely cooked through. Remove from heat and let cool to room temperature.
2. Blend or puree the plums. You should have about 2 cups (500ml) of plum puree.
3. Put the sugar, or sugar and corn syrup, in the saucepan with about 1/2 cup (125ml) of the plum puree and cook it over medium heat, stirring, until the sugar is completely dissolved. Remove from heat and stir it into the rest of the plum puree. Mix in the kirsch or maraschino and a few drops of lemon juice, then chill the mixture thoroughly, preferably overnight.
4. When the sorbet mixture is thoroughly chilled, freeze the sorbet in your ice cream machine according to the manufacturer’s instructions.