Start by making the pastry dough. Place the flour, salt, 2 tablespoons sugar, and chilled butter into a food processor and pulse until the mixture resembles coarse crumbs.
Gradually pour in the ice water, pulsing just until the dough comes together. If the mixture is still very dry and a dough is not forming, add a little more ice water.
Turn the dough out onto a lightly floured surface and shape it into a ball. Wrap it tightly in plastic wrap and refrigerate for at least 2 hours.
While the dough chills, prepare the blueberry filling. In a small saucepan over low heat, combine the blueberries, sugar, and cornstarch.
Stir frequently until the mixture thickens. Let it cool for about 30 minutes, then transfer it to a glass jar and refrigerate.
Take the dough out of the fridge 45 minutes before baking to let it soften slightly.
When you’re ready to assemble the tarts, preheat your oven to 180°C (356°F).
Roll the dough out with a rolling pin until it’s about 1/8 inch thick. This may take a little time, so be patient.
Cut the dough into 8 rectangles, approximately 3 x 4 inches each (you may get up to 10, depending on thickness of your dough).
Spoon 2 tablespoons of blueberry filling onto half of the rectangles. Place the remaining rectangles on top to form pastries.
Seal the edges by pressing them together with a fork. Lightly brush the tops with milk and sprinkle with a bit of sugar.
Bake for about 40 minutes, or until golden brown.
Remove from the oven and let the pop tarts cool for 20–30 minutes.
While they cool, prepare the icing by mixing powdered sugar with milk. This step is optional, but you can add a little blueberry juice to give the icing a light lavender colour.
Finish with sprinkles if desired, and enjoy!