2 pounds ripe Bartlett pears, peeled, cored, and cut into 1/4-inch-thick slices
1 cup sugar
4 teaspoons unflavored powdered gelatin from 2 (1/4-ounce) envelopes
2 cups heavy cream
2 cups whole milk
1/2 cup sugar
2 vanilla beans, split lengthwise
In large bowl, toss together pears and sugar. Cover and refrigerate at least 8 hours or overnight. In small bowl, sprinkle gelatin over cream. Let stand until gelatin softens, about 1 minute. Meanwhile, in medium saucepan over moderate heat, whisk together milk and sugar. Scrape in seeds from vanilla beans; add beans. Heat, whisking occasionally, until sugar dissolves, about 2 minutes. Remove from heat and whisk in cream mixture. Strain through fine-mesh sieve, discarding vanilla beans, then ladle mixture into 8 (4-ounce) ramekins. Cover and refrigerate at least 8 hours or overnight. In heavy medium saucepan over moderate heat, combine 1/2 cup water and pear mixture. Bring to simmer, uncovered, and cook until pears are tender, about 20 minutes. Transfer mixture to food processor and puree until smooth. Transfer jam to medium bowl, cover tightly, and refrigerate until cool, about 1 hour. (Jam will keep, covered, in refrigerator up to 1 week.) Run thin sharp knife around inside edge of each ramekin to loosen. Dip bottom of 1 ramekin in bowl of very warm water 6 seconds. Put plate over ramekin, then invert panna cotta onto plate, gently lifting off ramekin. Repeat to unmold remaining panna cottas. Top each with 1/4 cup pear jam and serve.