Preheat the oven to 200°C.
Peel the apples and then cut in half. Scoop out the core (a melon baller is useful here) and carefully cut away stem and base to leave as near perfect a half moon shape as possible.
Melt the butter in a 20cm / 8” Tarte Tatin tin (or an oven proof frying pan) over a medium heat.
Lower the heat and stir in the sugar until it starts to caramelise (about a minute).
Add the halved apples and cook gently for 10 minutes turning the apples to coat them in the caramel butter.
Remove the tin from the heat and with the apples cut side up, slot the blueberries in amongst the gaps.
Sprinkle 1tsp vanilla sugar over the apples.
Now unroll the puff pastry and prick all over with a fork.
Lay the pastry over the apples and trim with scissors leaving a 2cm excess.
Tuck the excess pasty in over the apples.
Cook on the middle shelf of oven for 25 minutes until the pastry is golden brown.
Place a serving platter on top and invert to serve, carefully removing tin.
Serve with a scoop of ice cream.