Finely chop the plums and soak in Armagnac.
Heat the milk with half of the sugar and the vanilla pod and leave to infuse for 20 minutes.
Beat the rest of the sugar with the egg yolks over a hot water bath until creamy.
Add the plums and the strained milk. Whip in ice water or in a food processor until cold, stirring constantly. Mix in the stiffly whipped cream.
Line a loaf pan with a freezer bag (cut open), pour in the ice cream mixture, cover and place in the freezer for 5 hours to set. To serve, turn out, remove the freezer bag and cut into slices. For the sauce, heat milk with sugar and vanilla bean, remove saucepan from heat source. Whisk egg yolks until smooth and stir in hot milk.
Return saucepan to hot plate and stir with a whisk until yolks bind.
Pour a dollop of sauce onto a plate and place three very thin slices of parfait. Serve garnished with a mint leaf.