Season the chicken breasts with salt and pepper.
In a large stock pot, combine the chicken stock and white wine and bring to a boil. Reduce the heat to a simmer, add the chicken breasts and poach for 15 to 20 minutes or until cooked through.
Remove the chicken from the poaching liquid; reserve the liquid. When cool enough to handle, shred the chicken.
In a large sauté pan, melt 2 tablespoons butter. Add the leeks and sweat until just tender.
In a large sauté pan, make a thick roux by melting 8 tablespoons butter, then adding the flour and stirring constantly until lightly golden.
Add the cream to the poaching liquid and bring to a boil. Whisk in the roux and reduce to low heat.
Add the leeks and shredded chicken and simmer for 20 minutes.
Preheat the oven to the called-for temperature on the box of puff pastry.
Melt the remaining 2 tablespoons butter.
Fill soup bowls to within ½ inch from the rim with hot Leekie.
Top each bowl with a square of puff pastry and brush each with melted butter.
Bake per the instructions on the puff pastry box or until the pastry is golden brown.
Serve hot.