Dilute cornstarch in water, set to side.

Wash and slice thinly the rhubarb. Cook them with sugar on medium heat, until sugar melted and rhubarb softened to mushy texture. Pour in cornstarch, cook til thickened. Cool to room temperature.

Preheat oven to 400 F

Cream cream cheese and sugar, reserve 1 tbsp mixture, then cream in cooled rhubarb, until completely combined.

Mix sour cream into reserved cream cheese, and spoon into piping bag.

Lay pastry and slice into squares.

Spoon rhubarb in center and shape pastry into parcel as shown.

Pipe sour cream on pastry.

Beat yolk with water, and brush mixture on each pastry, and bake pastry for 20-25 mins