Thaw pastry on bench and heat oven to 390 (200C) and line a cookie tray with parchment paper.
In a medium sized saucepan over a medium heat, melt the butter and cook down the garlic and leek for around 8 minutes until soft. Add chicken and cook through.
Add the wine and stock to the saucepan and continue to cook for 5 minutes. Season with salty and pepper.
In a small bowl mix together the cornflour and water to create a paste. Add this to the chicken mixture along with the chopped parsley. Mix through and remove from heat.
Take the first sheet of the puff pastry and place on a cutting board. Leaving a 1 ½ inch border around all sides make about 8 evenly spaced slits along the width of the pastry sheet.
Place the second sheet on the cookie tray and spoon the chicken mixture into the center. Spread the mixture out, leaving a 1 ½ inch border. Place the slices of brie on top. With a pastry brush (or your finger), brush some of the beaten egg around this border. Place the first sheet on top of the second, press down the border with your fingers and brush the egg over the top of the pie.
Cook for around 20 minutes until the pastry is golden brown. Let stand for 5 minutes before cutting.