Step 1: Prepare the Pastry

1. Preheat oven to 425°F (218°C).
2. If using frozen puff pastry, thaw according to package instructions until pliable but still cold. If using refrigerated, skip this step.
3. Lightly roll out the sheets with a rolling pin.
4. Cut each sheet into 4 equal squares (about 5 inches each), making 8 squares total.

Step 2: Shape and Bake the Croissants

5. Brush each square with egg wash, then fold into a triangle, sealing the edges.
6. Brush the tops with more egg wash and sprinkle with coarse sugar (optional).
7. Bake for 22-25 minutes until deep golden brown.
8. Set aside to cool completely before cutting.

Step 3: Make the Jam

9. In a saucepan over medium heat, combine blueberries, sugar, and lemon juice.
10. Bring to a simmer and cook for 15-20 minutes, stirring frequently, until thickened.
11. Remove from heat and cool.
    Note: If using fresh fruit, cook for an additional 5 minutes to reduce excess moisture.

Step 4: Make the Cream Filling

12. In a large bowl, beat cold heavy cream, powdered sugar, and vanilla extract until stiff peaks form.
13. In a separate bowl, beat cream cheese and sour cream until fluffy.
14. Gently fold the whipped cream into the cream cheese mixture, ⅓ at a time, until fully combined.
    Note: The filling can be made a day ahead and refrigerated. Bring to room temperature and re-whip before using.

Step 5: Assemble the Croissants

15. Slice each croissant in half, but not all the way through.
16. Spread a generous layer of berry jam on both halves.
17. Fill with cream filling and close the croissants.
18. Dust with powdered sugar before serving.