Preheat the oven to 200°C.
Roll out the pastry on a lightly floured surface into a 1cm thick rounds that will fill 6cm diameter fluted tart tins. Prick the bases and chill for 15 minutes.
Cut small rounds of nonstick baking paper that will fill the tart tins. Line with baking beans and bake the tartlets for 8-10 minutes until the edges start to colour. Remove from the oven and discard the baking beans and the nonstick baking paper.
Beat the egg with a little water to create an egg wash. Brush the tartlets with the egg wash and return to the oven for 4-5 minutes. Remove and cool on a wire rack. Meanwhile, prepare the pastry cream by whisking the 4 egg yolks with the sugar until pale and thick. Sift in the flour and whisk well.
Combine the vanilla pod and the milk in a sauce pan and bring to the boil. Pour the milk onto the egg yolk mixture whisking simultaneously until fully incorporated. Return the mixture to the saucepan and whisk gently over a medium-low heat until it comes to the boil and thickens. Cook for a further 2 minutes before removing from the heat and transferring to a piping bag with a straight nozzle.
Chill in the fridge until cold. Once cold, pipe into the base of the pastry case. Arrange the blueberries (making sure they are thoroughly defrosted) in half of the cases and raspberries in the other half. Dust the blueberry tartlets with icing sugar and serve when ready.