In a mixing bowl, combine the flour, sugar, and salt. Cut in the cold butter until the mixture resembles coarse crumbs.

Gradually add cold water, mixing just until the dough holds together.

Shape the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 30 minutes.

While the dough is chilling, mash the mixed berries and add a little sugar to enhance their flavor.

Once chilled, roll out the dough on a floured surface to about 1/8-inch thickness. Cut rectangles, roughly 3x4 inches.

Place a tablespoon of the berry filling on half of the rectangles. Brush the edges with a bit of egg wash, then place another rectangle on top. Seal the edges with a fork.

Preheat your oven to 375°F (190°C). Place the pop tarts on a baking sheet lined with parchment paper. Brush the tops with the remaining egg wash. Bake for about 20 minutes, or until golden brown.

Allow the pop tarts to cool slightly and then drizzle with icing made from powdered sugar and a little water.