Preheat your oven to 375˚F and line a baking sheet with parchment paper.
In a small saucepan over medium heat, combine blueberries, lemon juice, zest, cornstarch, and sugar. Stir well and gently mash the blueberries. Cook until the jam thickens (around 6 minutes). Transfer to a bowl and let it cool slightly.
Roll out the puff pastry and cut it into 8 squares (or 12 for smaller pop tarts).
Place 4 squares on the baking sheet. Fill each with about 1 1/2-2 tablespoons of blueberry jam (less if using smaller squares). Top with another puff pastry square and seal the edges with a fork.
Brush with egg wash (or non-dairy milk for a vegan option) and bake for approximately 18 minutes, or until the puff pastry turns golden brown.
Whisk together the powdered sugar and blueberry juice until smooth. Allow the pop tarts to cool, then drizzle them with the glaze and sprinkle on some lemon zest. Enjoy these delightful treats!