Line a 1 1/2 quart terrine mold by buttering it and fitting it with plastic wrap (the one I use has a removable bottom, pins to join the sides, straight walls and is about 2 1/2 by 2 1/2 by 12 inches). In a metal mixing bowl whisk together the yolks, sugar, milk, orange juice and split vanilla bean. Place over simmering water and continue to whisk until light, fluffy and hot. Remove from the heat and place in an ice bath to cool. Once it's cool, fold in 2 cups of whipped cream and the poppy seeds. Pour mixture into the terrine mold and freeze overnight.
To make the plum sauce remove the stones from the plums and place in a saucepan with the water and sugar. Bring to a boil and simmer until the plum skins split. Puree with a hand blender and strain. Chill.
Serve terrine in slices with drizzles of plum sauce and remaining 1/2 cup whipped cream.