Ingredient note: Lavender flowers impart a delicate fragrance to custards and ice creams. You can find dried culinary lavender in herb stores and specialty stores. If you don't have lavender, just leave it out: in Steps 1 and 2, simply heat the milk until steaming.You can also use the custard as a sauce for fresh raspberries or figs.
To prepare custard: Heat milk in a small heavy saucepan over medium heat until steaming. Remove from heat. Add lavender, cover and let steep for 30 minutes.
Strain milk through a fine sieve into a medium bowl. Return the milk to the saucepan and reheat until steaming.
Whisk egg yolks, honey and cornstarch in a medium bowl until smooth. Gradually add the hot milk, whisking until blended. Return the mixture to the saucepan. Cook over medium heat, whisking constantly, until slightly thickened and starting to bubble gently, 1 1/2 to 2 minutes. Transfer to a clean bowl. Whisk in vanilla. Cover loosely and refrigerate until chilled, about 1 hour.
To prepare fruit & topping: Preheat broiler. Coat an 11-by-7-inch oval gratin dish or 4 individual gratin dishes with cooking spray. Spoon custard evenly into the dish or dishes. Arrange plums on their sides, slightly overlapping, in a single layer over custard. Sprinkle sugar evenly over plums. Broil until plums are lightly caramelized, 5 to 7 minutes. Serve immediately.