Preheat oven to 200°C. Mix flour, hazelnut powder and sugar. Make a hole and add egg, oil and water. Knead dough. Roll out and fill in 6 tartlet molds. Prick with a fork. Cover with baking paper and fill with cooking beads. Bake for 8 minutes. Unmold and bake for another 4 minutes. Leave to cool.
Fill the tartlets with 2 teaspoons of double cream Gruyère (or more for gourmands) and then make vermicelli using the chestnut purée. You can finish with another teaspoon of double cream Gruyère and sour cherry !