This is from Gourmet September 2008. 1 1/2 tsp unflavored gelatin may be substituted for agar if you are unable to find the agar. Yield 4 servings Prep and cooking time do not include cooling time or maceration time.
Using a paring knife, scrape vanilla seeds from pod into a 1 1/2- to 2-quarts heavy saucepan, then add pod, milk, cream, and 3 tablespoons sugar. Bring to a boil. Whisk in agar and simmer, whisking, until completely dissolved, about 5 minutes. Discard pod. Cool panna cotta slightly. Pour into glasses and chill until set, at least 2 hours.
Meanwhile, peel mangoes and thinly slice.
Bring water, juices, and remaining 3 tablespoon sugar to a simmer in cleaned saucepan, whisking until sugar has dissolved. Pour over mangoes in a bowl and macerate until ready to use. Serve panna cotta with mangoes.