Meanwhile, line a strainer with cheesecloth. Fill bottom half of double boiler with water and bring to a simmer. Combine heavy cream and maple syrup in a small saucepan and bring to just a boil. Add cream and syrup mixture to buttermilk and gelatin mixture in top half of double boiler and place over bottom half with simmering water, whisking until gelatin is dissolved, about 4 minutes.
Stir in remaining 1 1/2 cups buttermilk. Pass through cheesecloth-lined strainer. Pour into 8 to 10 small custard cups or cordial glasses. Place plastic wrap on top of each container and refrigerate at least 5 hours.
Serve custard in cordial glasses, or unmold it onto plates by dipping bottom of each cup in hot water a few seconds and running tip of a knife around edge before turning out onto plate. Drizzle additional maple syrup on top if desired.