In a 2- to 3-quart pan over medium-high heat, bring cream, lavender, and vanilla to a boil, stirring often. Remove from heat; let stand 10 minutes.
In a bowl, beat yolks and 1/4 cup sugar to blend.
Pour cream mixture through a fine strainer into bowl with yolks. Scrape seeds from vanilla bean into the cream mixture; stir. Rinse bean and let dry for other uses. Discard lavender.
Pour cream custard mixture equally into 4 ovenproof crème brûlée dishes or ramekins (each about 1 in. deep and 5 1/2 in. wide). Set dishes in a 2-inch-deep 11- by 17-inch pan.
Set pan on rack of a 300° oven. Carefully pour boiling water into pan up to level of custard. Bake until custards jiggle only slightly when shaken gently, 15 to 18 minutes. With a wide spatula, carefully lift dishes from water onto a rack. Let desserts cool, then cover and chill until cold to touch, about 1 hour or up to 1 day.
In a 6- to 8-inch frying pan over medium heat, shake 2/3 cup sugar until melted and pale amber colored, 8 to 10 minutes. Working quickly, drizzle about a quarter of the hot sugar syrup over 1 custard at a time. Immediately tilt dish so syrup flows evenly over custard. Syrup hardens almost at once. Repeat to glaze remaining custards. Serve, or let desserts chill up to 1 hour so the glaze dissolves a little and gets thinner.