In the bowl of a food processor, combine the flour, sugar and salt. Add the cubed butter and pulse until combined. With the motor running, slowly stream in the ice water, 1 tablespoon at a time, until the dough begins to form a ball. (Alternately, the dough can be made by hand by cutting the butter into the flour with your fingers then stirring in the ice water.)
Divide the dough in half, shaping each piece into a flattened disc. Wrap each disc securely in plastic wrap. Refrigerate the dough for 15 minutes.
Lightly flour your work surface then using a rolling pin, roll out each piece of dough into a 16-by-20-inch rectangle that’s roughly 1/8-inch thick. Use a ruler to measure out eight 4-inch-by 5-inch rectangles.
Divide filling of choice evenly between 8 rectangles.
Brush the egg wash around edges and cover with the other 8 pastry rectangles. Crimp edges with a fork. Chill in the fridge for at least 30 minutes.
Preheat the oven to 350°F.
Brush chilled pop tarts with remaining egg wash. Bake for 20 minutes and check - pop tarts should be golden brown.
Allow to cool completely while you make the glaze.