Preheat oven to 400 degrees F. Coat four 1- to 1 1/2-cup gratin dishes or a shallow 1-quart baking dish with cooking spray.
Toss plums with sugar and amaretto, if using, in a medium bowl. Spread evenly in prepared dish(es). Bake, uncovered, until tender, about 20 minutes.
Meanwhile, whisk egg yolk, yogurt, condensed milk and vanilla in a medium bowl until smooth. Cover and set aside in the refrigerator until needed.
Shortly before serving, preheat broiler. Spoon the yogurt mixture over plums. Broil 4 to 6 inches from heat until golden and bubbly, about 3 minutes. Sprinkle with cookie crumbs. Serve hot or warm.
Make Ahead Tip: The plums (though Step 2) will keep, loosely covered, at room temperature for up to 2 hours.
1 fruit, 2 other carbohydrate