Preheat the 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 (about 6 minutes). Transfer to a bowl and let it cool slightly.
Roll out the puff pastry and cut it into 8 even squares (or 12 for smaller pop tarts).
Place 4 squares on the baking sheet. Fill each with approximately 1 1/2-2 tbsp of blueberry jam (use less for smaller squares), then top with another puff pastry square. Seal the edges with a fork.
Brush with egg wash (or nondairy milk for a vegan option) and bake for around 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 generously top them with the glaze and optional lemon zest. Enjoy these delightful treats!