1. In a large mixing bowl, cut the flour and butter together with a pastry cutter or two knives. Add the sugar and salt and mix well. Add ½ cup of cold water and mix until just combined into a dough. If needed, add extra water, 1 teaspoon at a time. Wrap the dough in plastic wrap and refrigerate for at least half an hour.
2. In a small mixing bowl, combine the filling ingredients. Preheat the oven to 375 degrees.
3. Roll out the pastry dough, as thin as possible, then cut with a sharp knife into rectangles (you will probably have to do this in batches). Brush half of the rectangle with the beaten egg, then top the center of the brushed rectangles with a generous amount of the filling mixture (don’t spread all the way to the edges). Top with the un-brushed rectangles. Using a fork, press all the sides of the tarts, sealing the edges. Prick the top of the tarts a few times with a fork, to allow venting. Brush with the beaten egg.
4. Line a baking sheet with parchment paper (or grease with cooking spray). Place the tarts on the prepared sheet, and bake for approximately 15 minutes, or until lightly golden.
5. Combine the icing ingredients, adding a little more milk if necessary. Spread the icing on tarts. ENJOY!