Preheat oven to 180C
Mix mascarpone, sugar, and vanilla together
Slice pastry in half lengthways
Spread mascarpone mixture on one side and spread the berries over it
Fold the other half up lengthways and make slits in it
Cover over the other half and press down the edges with a fork
Brush with egg wash and sprinkle with sugar
Bake for 20 minutes at 180C
Turn down to 170C and bake for another 10-15 minutes
