Prepare the Croissants:

1. Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.
2. Roll out the pre-made puff pastry sheets on a lightly floured surface. Cut each sheet into triangles (or squares if you prefer a different shape).
3. Roll the triangles from the wider end to the tip to form croissants.
4. Brush each croissant with the whisked egg to give them a golden, shiny finish during baking.
5. Bake in the preheated oven for 12-15 minutes, or until the croissants are puffed and golden brown.

Make the Berry Jam:

1. In a medium saucepan, combine the frozen wild blueberries, granulated sugar, and lemon juice.
2. Cook over medium heat, stirring occasionally, until the berries break down and the mixture thickens into a jam-like consistency, about 8-10 minutes.
3. Remove from heat and let the jam cool to room temperature.

Prepare the Cream Filling:

1. In a large mixing bowl, whip the heavy whipping cream until soft peaks form. Set aside.
2. In another bowl, beat the softened cream cheese, sour cream, powdered sugar, and vanilla extract together until smooth and creamy.
3. Gently fold the whipped cream into the cream cheese mixture until fully combined and light.
4. Taste and adjust the sweetness if necessary by adding more powdered sugar.

Assemble the Croissants:

1. Once the croissants have cooled, carefully slice each croissant in half horizontally.
2. Spread a layer of the prepared cream filling on the bottom half of the croissant, then spoon a layer of the berry jam on top.
3. Place the top half of the croissant back on and dust with powdered sugar before serving.

Serve:

1. Serve the croissants immediately for the freshest flavor and texture.
2. They can be enjoyed as a decadent breakfast treat or an elegant dessert.