How to Make It
Whisk together first 4 ingredients and 1 1/3 cups sugar in a medium saucepan. Cook over medium-low heat, whisking often, 10 to 12 minutes or until mixture just begins to boil. Pour mixture into a medium-size metal bowl. Fill a large bowl halfway with ice water. Place bowl containing custard mixture in ice water, and cool completely (about 30 minutes), whisking occasionally. Cover and chill custard mixture 2 hours.
Meanwhile, stir together plums, corn syrup, salt, and remaining 3 Tbsp. sugar in a small saucepan, and cook over medium heat, stirring occasionally, 12 minutes or until plums are very tender. Remove from heat; let stand 15 minutes. Stir in lemon juice. Cover and chill plum mixture 2 hours.
Pour custard mixture into freezer container of a 2-qt. electric ice-cream maker, and freeze according to manufacturer's instructions. (Instructions and times may vary.) Transfer ice cream to a freezer-safe bowl or pan, and freeze 1 hour. Meanwhile, freeze plum mixture 30 minutes. Dollop frozen plum mixture over frozen ice cream, and gently swirl.
We tested with Cuisinart Pure Indulgence Frozen Yogurt-Sorbet & Ice Cream Maker.