In a bain-marie, gently whisk the egg yolks with one-third of the sugar until the mixture becomes a light, foamy cream.
In a saucepan, combine the milk, rosehip purée, and the remaining sugar. Bring to a boil. Gradually pour the hot milk mixture into the egg yolk cream, whisking continuously.
Return the mixture to the saucepan and cook over low heat, stirring constantly, until it thickens enough to coat the back of a spoon. Avoid boiling to prevent curdling. Let cool, then fold in the sweetened whipped cream.
Strain the cooled custard through a sieve and churn in an ice cream maker according to the manufacturer's instructions.
For the biscuits, separate the eggs. Whisk the yolks with hot water until thick and pale. Beat the egg whites to stiff peaks and gently fold into the yolk mixture. Sift in the flour, starch, and melted butter, folding carefully to combine.
Grease a baking sheet, line with parchment paper, and spread the batter 1 cm thick. Bake in a preheated oven at 180°C (350°F) for 10 minutes, or until golden.
Mix the yogurt with one-third of the sugar and the lemon juice.
Rinse and dry the blueberries. Whip the cream to soft peaks. Toss the blueberries with the remaining sugar, then fold in the whipped cream followed by the yogurt mixture.
Cut the cooled biscuit into rounds. Top with the yogurt-blueberry cream and fold into half-moons.
Serve the warm biscuits with a scoop of rosehip ice cream, garnished with fresh seasonal fruits.
