Use red-skinned, deep-purple, or black-skinned plums in order to give this ice cream a delicate rosy hue.
Combine the plums and sugar in a medium saucepan; cook over medium heat 5 minutes or until plums are tender and sugar is dissolved. Place plum mixture in a food processor; process until smooth. Strain plum mixture through a sieve into a bowl to measure 2 1/4 cups puree, and discard solids. Cover and chill completely.
Combine 1 1/2 cups plum puree and milk in the freezer can of an ice-cream freezer; freeze according to manufacturer's instructions. Spoon ice cream into a freezer-safe container; cover and freeze until firm (about 1 hour).
Combine 3/4 cup plum puree and cognac in a small bowl. Spoon over ice cream, and sprinkle with slivered almonds.