Preparation Lightly oil 10 (1/2-cup) ramekins and set aside. In medium heatproof bowl set over pot of simmering water, heat 1 1/2 cups buttermilk and gelatin, stirring constantly, until gelatin melts, about 3 minutes. Remove from heat and set aside. In heavy, large saucepan over moderately low heat, whisk together cream and sugar. Bring to simmer and cook, stirring often, until sugar is dissolved, about 7 to 9 minutes. Slowly whisk buttermilk-gelatin mixture into cream mixture, then whisk in remaining 1 1/2 cups buttermilk. Divide mixture among ramekins, cover, and chill until set, at least 2 hours or overnight. When ready to serve, dip ramekins, 1 at a time, into bowl of hot water for 3 seconds. Run thin knife around edge of each ramekin and invert ramekin onto center of shallow bowl. Shake gently to release panna cotta. Top each with 2 tablespoons strawberry jam and garnish with sprig of mint.