Preheat oven to 400 degrees. Line a baking sheet with parchment paper.
In a saucepan over medium heat, add blueberries, lemon juice, cornstarch and sugar. Simmer, stirring occasionally, until you have a nice blueberry sauce. Don't mush the berries. Remove berry sauce from heat when done and let cool.
While the berries are simmering, add cream cheese, lemon juice, egg yolk and vanilla extract to the bowl of your stand mixer. With the paddle attachment, beat until smooth and fluffy.
Roll pastry dough out into a 8"x16" rectangle. Cut into 8 equal pieces. Transfer to parchment paper lined baking sheet.
Evenly distribute cream cheese mixture among the 8 pastries by placing a dollop in the center of the puff pastry dough. Gently spread cheese mixture leaving about a half inch around the edges. Top cheese with about 1-2 tablespoons of blueberry topping.
In a small bowl, whisk egg.
Fold edges of the pastry and brush edges with egg wash. Sprinkle egg washed edges with sugar.
Bake for 18-22 minutes until edges are golden. Remove from oven and transfer to a cooling rack.