Preheat the oven to 375°F.
In a medium bowl, whip together mascarpone cheese, lemon juice, powdered sugar and vanilla using a hand blender. Set aside.
Cut pastry into approximately 3.5 inch squares (18 squares). Working in batches, use a sharp knife, cut two ‘arrows’ on opposite sides of the square leaving two sides attached to the square. Flip the cut pieces across each other forming a diamond shape in the centre. Place pastry on a parchment lined baking sheet. Fill each square with a tablespoon of mascarpone filling and top with a handful of berries.
Let the pastry chill in the fridge or freezer for 10 minutes prior to baking. Brush with egg wash and bake for 28-30 minutes until golden brown.