In a small saucepan over medium heat stir the berries, lemon juice, sugar, and cornstarch. Bring to a boil then reduce to a simmer and cook for 6-7 minutes, stirring often. Remove from the heat and stir in the vanilla extract. Let cool completely.
Preheat the oven to 425°F. Line a baking sheet with parchment paper.
Cut the puff pastry sheets into 6 rectangles per sheet for 12 rectangles total.
Spread 6 of the puff pastry rectangles on the parchment paper. Pierce the bottom of each rectangle with a fork (so that it doesn’t puff up). Fill the centers with a few spoonful of the berry mixture, leave a 1/2"-1" inch border on all sides. Use a pastry brush and brush the edges of the puff pastry with water. Use the remaining half of the puff pastry rectangles to top the berry covered pastries. (Do your best to line them up so they’re even).
Use the tines of a fork to seal all 4 sides of the pastry. Poke a couple holes along the top of the pastry with the fork. Brush each pop tart with the egg wash.
Bake for 20 minutes or until golden brown. Remove from the oven and let cool 10 minutes before topping them with the glaze.