Preheat oven to 350 Degrees Fahrenheit. Roll out puff pastry dough into a 10 inch by 10 inch square. (I prefer to use rolled pastry doughs because it gives you a perfect even square to begin with!)
Using a sharp knife, cut dough horizontally down the middle, creating 2 rectangles. Then cut dough vertically in four equal parts. (If you're using a 10x10 rectangle you'll make the cuts every 2 1/2 inches.) Top 4 of your rectangles with one tbsp of jam, leaving a small border of pastry around the jam. Repeat with other puff pastry sheet.
Place the remaining pastry on top of the jam filled pastry. Gently secure edges with your fingers and using a fork, crimp edges.
Whisk together 1 tbsp of milk and egg. Brush tops of pastry with egg wash and bake for 15-18 minutes.
While baking, prepare your icing. Whisk together sugar, remaining milk and vanilla until well incorporated. If icing is too thick, add a touch more milk.
Remove poptarts from oven and let cool for 10-15 minutes. Top with icing and sprinkle on cereal. Serve and Enjoy!