Preheat your oven to 400°F (200°C).
Let the puff pastry sit at room temperature for about 10 minutes.
Cut the puff pastry into four 4-inch squares and arrange them on a baking sheet.
Add 1 tablespoon of mixed berry jam to the center of each square.
Fold the corners of the pastry over the jam and press down the edges with a fork to seal.
Brush melted butter over the top of each puff.
Sprinkle coarse sugar on top of each pastry.
Bake for 15-20 minutes, or until golden brown, keeping an eye to avoid burning.
Let cool on a wire rack for a few minutes before serving.
