In a mixing bowl, combine the flour, sugar, and salt. Add the cold, cubed butter and mix until you have a crumbly texture.

In a separate bowl, whisk the egg and add it to the flour mixture. Gradually add cold water, one tablespoon at a time, until the dough comes together.

Wrap the dough in plastic wrap and let it chill in the refrigerator for 30 minutes.

Preheat your oven to 375°F (190°C). On a lightly floured surface, roll out your chilled dough to about 1/8 inch thickness. Cut into rectangles (about 3x5 inches).

Place half of the rectangles on a baking sheet lined with parchment paper. Add a tablespoon of your chosen filling to the center of each rectangle, leaving a small border.

Place the remaining rectangles on top of the filled ones, pressing the edges firmly to seal. Use a fork to crimp the edges for a decorative touch.

Bake in the preheated oven for 20-25 minutes, or until golden brown. Remove and allow them to cool slightly.

To make the icing, whisk together powdered sugar and a little milk until you reach a spreadable consistency. Drizzle over the cooled pop tarts.