Preheat oven to 400 degrees. Line a baking sheet with parchment paper. Set aside.
Add cornstarch to raspberry jam in a small bowl and mix together until cornstarch is not longer visible. Set aside. Roll out one sheet of puff pastry on a floured surface until it measures 11.5" x 9.5". Using a fork, poke little holes all throughout the pastry. Cut pastry into 6 even rectangles.
Place three rectangles onto the prepared baking sheet. Spoon 1.5 tablespoons of jam into the center, leaving about 3/4-inch clean around the edge of the pastry rectangle. Take egg and milk and whisk together in a small bowl to make an egg wash. Brush the egg wash around the clean edge of the pastry. Take one of the remaining three clean pastry rectangles and place onto one of the jam pastries on the baking sheet. Use a fork to seal the edges. Brush the top of the pastries with egg wash. Repeat with the second sheet of pastry.
Bake in preheat oven for 20 minutes or until the pastry is golden brown on top. Remove from the oven and let cool for 10 minutes. Mix the icing ingredients in a bowl until smooth. Transfer to a pastry bag or storage bag. Snip the tip or corner of the bag. Drizzle icing over the slightly cooled pastry in a zig-zag pattern. Let set for 5 minutes and then serve.