Make the filling: Add strawberries, sugar, lemon juice, and cornstarch to a small saucepan. Cook over medium heat, stirring, until thick and jammy, 6–8 minutes.
Stir in vanilla, cool completely.
Start the dough: In a bowl, whisk flour, sugar, and salt. Cut in the cold butter with a pastry cutter or your fingertips until pea-sized bits remain.
Bind the dough: Whisk the egg with 3 tablespoons ice water. Add to the flour mix and stir until it clumps.
If dry, add more ice water, 1 teaspoon at a time, just until it holds together.
Chill: Divide dough in half, shape into rectangles, wrap, and chill 30–45 minutes. Cold dough is key for flakiness.
Roll it out: Dust parchment with gluten free flour. Roll one portion to about 1/8 inch thick.
Trim edges to form a neat rectangle.
Cut rectangles: Cut 3x4-inch rectangles. Repeat with remaining dough. You should get 12 rectangles for 6 pop tarts, depending on thickness.
Fill: Place half the rectangles on a parchment-lined sheet.
Brush edges with egg wash. Spoon 1–1 1/2 tablespoons strawberry filling in the center, leaving a border.
Seal: Top with remaining rectangles. Press edges to seal, then crimp with a fork.
Prick the tops 3–4 times with a fork to vent.
Chill again: Freeze the tray for 10 minutes while you preheat the oven to 375°F (190°C). Cold pastries bake cleaner and hold shape.
Bake: Brush tops with egg wash. Bake 18–22 minutes, until golden at the edges.
Cool on a rack completely before glazing.
Glaze: Whisk powdered sugar, milk, vanilla, and a pinch of salt until smooth and thick but pourable. Spoon over cooled pastries and let set 15 minutes.