1. In a small bowl, sprinkle gelatin over water; let soften 8 to 10 minutes.
2. In a medium saucepan, combine milk and sugar. Bring to a simmer, reduce heat to low and cook, stirring constantly, until sugar is dissolved, about 4 minutes. Remove from heat and whisk in softened gelatin, buttermilk and vanilla extract. Strain through a fine-mesh sieve and divide among 12 small juice glasses or ramekins. Cover with plastic wrap and chill 3 hours or overnight.
3. In a large bowl, toss together pineapple, mango, orange and orange juice. Top each panna cotta with fruit mixture and pomegranate seeds.