Boil milk, cream, vanilla pod, and seeds in a saucepan.
Whisk egg yolks and caster sugar in a bowl and pour in the hot milk mixture.
Simmer the mixture in a saucepan until thickened while stirring constantly.
Reserve one quarter of the custard for hazelnut custard and three quarters for the parfait.
Blanch peppermint leaves in boiling water for 30 seconds then refresh in iced water.
Blend the larger portion of custard with mint leaves and essence, then strain and cool.
Grease a loaf tin and line it with two layers of cling film.
Whisk egg whites to soft peaks, fold in sugar, then whisk into the mint custard.
Fold whipped cream and grated chocolate into the mixture and freeze in the tin for four hours.
Melt sugar in a frying pan until golden-brown, stir in hazelnuts, and cool on a tray.
Crush the hardened hazelnut praline into a fine powder and stir into the reserved custard.
Melt chocolate with demerara sugar and mint essence, let it set on a tray, and break into shards.
Serve thick slices of parfait with the hazelnut custard and chocolate shards.