Preheat your oven to 400°F (200°C) and line two baking sheets with parchment paper. Keep the puff pastry refrigerated until you're ready to work with it.
In a medium bowl, beat the cream cheese, granulated sugar, and vanilla extract with a hand mixer until smooth and fluffy, about 2 minutes. Set aside.
In a small bowl, toss the diced strawberries with strawberry jam and cornstarch until the berries are evenly coated. This mixture will thicken as it bakes and prevent leaking.
Unfold one sheet of puff pastry on a lightly floured surface and cut it into 4 equal squares. Repeat with the second sheet to make 8 squares total.
Place about 1 tablespoon of cream cheese mixture in the center of each square, leaving a 1/2-inch border. Top with 1 tablespoon of the strawberry mixture.
Fold each square diagonally to form a triangle, pressing the edges firmly with a fork to seal. Use the fork tines to crimp a decorative border.
Transfer the croissants to your prepared baking sheets, spacing them 2 inches apart. Brush the tops with egg wash, avoiding the sealed edges, then sprinkle with coarse sugar.
Bake for 18-22 minutes, rotating the pans halfway through, until the croissants are deeply golden and puffed. The pastry should look crisp and the edges slightly darker.
Let the croissants cool on the baking sheets for 5 minutes before transferring to a wire rack. The filling will be very hot initially but will set as it cools.
Dust with powdered sugar just before serving if desired. These are best enjoyed warm or at room temperature within a few hours of baking.
