In a medium bowl, whisk together the flour, sugar and salt. Add the cubed butter, making sure each cube is completely covered in flour. Squeeze each piece between your fingers, continue doing this until the butter is the size of walnut halves. Mix in the egg and milk, combining until there are no more dry pockets. Split in half and wrap each half in clingfilm. Refrigerate for at least 30 minutes.
Make Filling
In a small saucepan, combine the jam, cornstarch and water together over medium heat. Bring to a boil for 2 minutes. Remove from heat and add the vanilla, then let cool.
Assemble
Place one half of the dough in a floured work surface and roll into a rectangle that is 9x12". Using a pizza cutter, cut the dough in 3 inch intervals so you have 4 individual strips of dough.
Beat the egg and milk together. Brush over the strips of dough. Place one tablespoon of filling on each strip towards one end, but leaving at least ¼ inch border. Fold the top over so the two edges meet and seal them with your fingers. Once sealed, take a fork and gently press the tines along all 4 edges. You can take a knife and trim the edges to make them straight.
Repeat with the other half of the dough.
Place the pop tarts on a parchment lined baking tray and prick the top of each pop tart with a fork. Refrigerate uncovered for at least 30 minutes.
Preheat the oven to 350°F/175°C.
Remove the tarts from the fridge and brush them with the remaining egg wash. Bake for 30-35 minutes, until a light golden brown. Allow them to cool completely.
Make icing
In a small bowl, mix icing sugar with one teaspoon of water. Mix with a spoon and continue adding water, a teaspoon at a time, until the icing is thick but pourable.