If frozen, allow puff pastry to thaw at room temperature.
Cook chicken breasts in boiling water with salt, pepper, bay leaves, a cinnamon stick, and a chicken stock cube until soft. Once cooked, shred using a fork.
Preheat the oven to 200°C/390°F, and lightly spray a 9x13 inch pan with baking spray and line with a sheet of puff pastry.
In a medium-sized pan or pot on medium heat, melt the butter. Add the onion and carrot, sauté until tender.
Sprinkle flour over vegetables and cook for an additional minute.
Whisk in milk, cream, and 1/2 cup of chicken broth (from the cooked chicken). Bring to a boil, then let it simmer on medium heat for about 5 minutes or until sauce begins to thicken.
Reduce heat and add mushrooms, corn, salt, and pepper.
Pour the chicken mixture over the puff pastry, then top with a second sheet of puff pastry (consider creating a lattice pattern for extra style).
Beat the egg and brush it over the top.
Bake for 40 minutes or until the top is golden brown and looks done.