Make the Pastry Dough:
In a large bowl, mix flour, sugar, and salt.
Cut in the cold butter with a pastry cutter or fork until mixture resembles coarse crumbs.
Beat the egg and add to the dough. Add ice water one tablespoon at a time, mixing until dough comes together.
Divide into two discs, wrap in plastic wrap, and chill for 30 minutes.
Prepare the Filling:
In a small bowl, mix the cornstarch and water until dissolved.
In a small saucepan over medium heat, combine strawberry jam with the cornstarch mixture and bring to a simmer. Cook for 2 minutes, stirring constantly until thickened.
Let it cool completely.
Assemble the Pop-Tarts:
Preheat oven to 375°F (190°C). Line a baking sheet with parchment.
Roll out each dough disc to about 1/8 inch thickness. Cut into 3x4 inch rectangles (you should get 16).
Place 8 rectangles on the baking sheet. Spoon 1 tablespoon of strawberry filling into the center, leaving a ½ inch border.
Beat the second egg and brush edges with egg wash.
Place another rectangle on top, press edges with a fork to seal.
Prick the tops with a fork to allow steam to escape.
Brush tops with egg wash.
Bake:
Bake for 22–25 minutes or until golden brown.
Cool on the baking sheet for 5 minutes, then transfer to a wire rack.
Glaze and Decorate:
In a small bowl, mix powdered sugar, vanilla, and milk until smooth.
Spoon or drizzle over cooled tarts.
Add sprinkles if desired. Let glaze set before serving.